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* LASERS: SAFETY, DIODE LASERS, HELIUM NEON LASERS, DRIVE, INFO, PARTS *
* *
* A Practical Guide for Experimenters and Hobbyists *
* *
* **** Version 2.62 **** *
* *
* Copyright (C) 1994,1995,1996,1997 *
* Samuel M. Goldwasser *
* Corrections or suggestions to: sam@stdavids.picker.com *
* *
* --- All Rights Reserved --- *
* *
* Reproduction of this document in whole or in part is permitted *
* if both of the following conditions are satisfied: *
* *
* 1. This notice is included in its entirety at the beginning. *
* 2. There is no charge except to cover the costs of copying. *
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TABLE OF CONTENTS
Introduction - Scope of this document, related information.
Laser Safety - Hazards to vision, other issues, 100W light bulb versus
1 mW laser, safety classifications.
Diode Lasers - Basic considerations, visible and IR (e.g., from CD player)
types, testing, visibility, collimation.
Diode Laser Power Supplies - Drive requirements, modulation, sample circuits
for low and high power devices.
Helium Neon Lasers - Theory (simple), operation, sealed HeNe tubes, testing,
problems, collimation, recharging.
Helium Neon Laser Power Supplies - Tube requirements, PS approaches,
regulation and modulation circuits.
Complete HeNe Laser Power Supply Schematics - 7 AC line and 3 inverter types,
most you can build.
Items of Interest - General Laser Information - Laser power meters, speckle,
Fabry-Perot and DFB, more.
Laser Information Resources - Books, magazines, links.
Laser Parts Sources - Walk-in, mail order, high quality, surplus.
SCOPE OF THIS DOCUMENT:
Both laser diodes and helium neon (HeNe) lasers are popular hobbyist projects.
This document includes information on both - hopefully it will grow in the
future.
Our emphasis is on the care and feeding of these types of lasers. Thus, you
will not find much information on the design of laser shows or holography
experiments. I leave these to the many excellent books and articles that have
been published over the years. However, on-line resources for driving laser
diodes and powering helium neon lasers seem to be scarce. Some of those that
exist are incorrect and potentially dangerous (or at least destructive).
This document was written in the hopes of rectifying this situation.
In addition to basic information, there are 6 sample circuits for driving
laser diodes, 10 *complete* schematics for helium neon laser power supplies,
as well as simple modulators and other useful goodies. Most of these have
been tested and/or came from working commercial designs.
There are also pointers to other web resources, mail order suppliers of laser
parts and equipment, and references on lasers in general.
Contributions are always welcome.
Note: where another 'document' title is referenced without identification of
a specific book or paper, it will be located at the Sci.Electronics.Repair FAQ
web site:
http://www.paranoia.com/~filipg/REPAIR/.
DISCLAIMER:
This document is still under development. Many of the circuits have been
reverse engineered - traced from various schematics or actual hardware. There
may be errors in transcription, interpretation, analysis, or voltage or
current values listed. They are provided solely as the basis for your own
designs and are not guaranteed to be 'plans' that will work for your needs
without some tweaking. We are not responsible for damage to equipment, your
ego, blown parts, county wide power outages, mini black holes, planetary
disruptions, or personal injury that may result from the use of this material.
ACKNOWLEDGEMENTS:
Thanks to Don Klipstein (email: don@misty.com) for his comments and additions to
this document. His Web site (http://www.misty.com/~don/) is a valuable resource
for information relating to lighting and related technology in general.
RELATED INFORMATION:
* See the document: "Notes on the Troubleshooting and Repair of Compact Disc
Players and CDROM Drives" for more info on how the laser diodes in CD players
and CD ROM drives worked originally.
Where the manufacturer and part number for your laser diode are known, by all
means take advantage of the extensive applications information that is likely
to be available. Driving laser diodes without blowing them out is often not
easy - even for an experienced design engineer!
* See the document: "Various Schematics and Diagrams" for a variety of circuits
that may be useful in generating the high voltage for belium neon lasers (in
addition to those found in the chapter: "Complete Helium Neon Laser Power
Supply Schematics".
* See the chapter: "Laser Information Resources" for books, magazine articles,
and links to other laser related web sites.
LASER SAFETY
You only received one set of eyeballs?
Lasers have tended to be high glamor devices popular with with hobbyists,
experimenters, entertainers, and serious researchers alike. However, except
for very low power lasers - those with less than a fraction of a mW of beam
power - they do pose some unique hazards particularly with respect to instant
and permanent damage to vision.
There are several reasons for this even for lasers which do not represent any
sort of burning or fire risk:
* The output of many lasers is a parallel - collimated - beam which means
that not only is the energy concentrated in a small area but the lens of the
eye will focus it to a microscopic point on the retina instantly vaporizing
tissue in much less than the blink of an eye. A collimated beam represents
the rays from an object at infinity so if your eye is focused for distance,
the laser will be in focus as well - to a microscopic point.
The output of a laser pointer or helium neon laser is a collimated beam.
Even at power levels considered relatively safe, one shouldn't deliberately
stare into the beam for any reason. For these relatively low power lasers,
permanent eye damage is not that likely but why take chances? For these
lasers, viewing the spot projected on a white surface is perfectly safe.
* An output of 1 mW may not sound like much compared to a 100 W light bulb
but consider:
A 100 W light bulb puts out about 2 or 3 W of visible light (the rest is
mostly IR and heat) more or less uniformly distributed in all directions.
However, at any reasonable distance from the light bulb, the power density
(e.g., W/sq. mm) is much lower than for a collimated laser beam of even very
low power. And, it takes significant effort to produce any sort of truly
collimated beam from such a non-point source such as is present with even
the filament of a clear light bulb.
For example, at 10 cm from a 100 W bulb (which would be a very uncomfortable
place to be just due to the heat), the power density assuming 3 total watts
of light would be only about .025 mW/sq. mm. At 1 m, it would be only
.00025 mW/sq. mm or 250 mW/sq. m. Based on this back-of-the-envelope
calculation, a 10 mW laser beam spread out to a circular area .2 m in
diameter will be brighter than the 100 W light bulb at 1 m! And, close to
the laser itself, that beam may be only 1 *mm* in diameter and 40,000 times
more intense!
A popular graveyard joke in the laser industry is: "Do not stare into the
beam with your remaining good eye". Nonetheless, laser safety is no laughing
matter.
The most common types of lasers generally available to hobbyists - CD laser
diodes, visible laser diodes, laser pointers, and small HeNe lasers, are all
rated Class II or IIIa. See the section: "Laser safety classification".
Class II lasers should be relatively low risk if even minimal precautions are
taken. However, Class IIIa lasers must be taken much more seriously if the
beam is collimated - as it would be from a laser pointer or HeNe laser tube.
In addition, with helium neon lasers, high voltage power supplies are involved
so there is the added shock hazard resulting from touching or accidentally
coming in contact with uninsulated connections. See the document: "Safety
Guidelines for High Voltage and/or Line Powered Equipment" before working on
any type of equipment which uses line voltage or produces high voltage. Most
of these are quite low power so the actual risk of electrocution from the high
voltage side is relatively small but there may be AC line voltage involved and
there can be collateral damage from a reflex response to the shock. In
addition, a homemade power supply, in particular, may use components which are
grossly oversized for the application (due to low cost availability) like a
15,000 V, 400 W neon sign transformer even though only under 10 W of power is
actually needed (we definitely do NOT recommend this approach).
Furthermore, you may come across a truly high power CO2 or argon ion laser, or
even a 50 mW helium neon tube. These, rated Class IIIb or Class IV, represent
much more significant risks of both instant permanent eye damage even from
momentary reflections from shiny (specular) surfaces as well a very real fire
hazard. In addition there is a very real danger of electrocution from the high
voltage high current power supplies used to power these beasts. Since this
document does not deal with these types of lasers, the essential additional
precautions that must be taken are not covered. However, you must handle them
properly for your own safety and the safety of others around you and your
surroundings.
The following very large number is designed to impress: The power density
of a 1 mW laser beam when focused to a spot of around 2 um (which isn't
difficult with a simple convex lens) is around 250,000,000 W per square meter!
Be Extremely Careful When Working with any laser!
LASER SAFETY CLASSIFICATION:
(From: Richard Trotman (trotman@udel.edu)).
I'm paraphrasing from "Introduction to Lasers", C.O.R.D., 1990:
Class I -- EXEMPT LASERS, considered 'safe' for intrabeam viewing. Visible
beam.
Maximum power less than 0.4 uW.
Class II -- LOW-POWERED VISIBLE (CW) OR HIGH PRF LASERS, won't damage your
eye if viewed momentarily. Visible beam.
Maximum power less than 1 mW for HeNe.
Class IIIa -- MEDIUM POWER LASERS, focused beam can injure the eye.
HeNe power 1.0 to 5.0 mW.
Class IIIb -- MEDIUM POWER LASERS, diffuse reflection is not hazardous,
doesn't present a fire hazard.
Visible Argon laser power 5.0 to 500 mW.
Class IV -- HIGH POWER LASERS, diffuse reflection is hazardous and/or a fire
hazard.
The classifications depend on the wavelength of the light as well.
DIODE LASERS
INTRODUCTION:
Note: throughout this document, we will use the terms "laser diode" and "diode
laser" somewhat interchangeably.
Diode lasers use nearly microscopic chips of Gallium Arsenide or other exotic
semiconductors to generate coherent light in a very small package.
Laser diodes are solid state devices not all that different from LEDs. The
first laser diodes were developed quite early in the history of lasers but it
wasn't until the early 1980s that they became widely available - and their
price dropped accordingly. Now, there are a wide variety - some emitting
*watts* of optical power. The most common types found in common devices like
CD players and laser pointers have an output in the 3 to 5 mW range.
However, unlike LEDs, laser diodes require much greater care in their drive
electronics or else they *will* die - instantly. See the sections on CD and
visible laser diodes, below, before attempting to power or even handle them.
In their favor, laser diodes are very compact - the active element is
about the size of a grain of sand, low power (and low voltage), efficient
(especially compared to the gas lasers they replaced), rugged, and long
lived if treated properly.
They do have some disadvantages in addition to the critical drive requirements.
Optical performance is usually not equal to that of other laser types. In
particular, the coherence length and monochromicity are likely to be inferior.
This is not surprising considering that the laser cavity is a fraction of a
mm in length formed by the junction of the III-V semiconductor between cleaved
faces. Compare this to even the smallest common HeNe laser tubes with about
a 10 cm cavity. Thus, most laser diodes would not be suitable light sources
for holography or interferometry, for example.
However, for many applications, laser diodes are perfectly adequate and their
advantages especially small size, low power, and low cost - far outweigh any
faults. In fact, these 'faults' can prove to be advantageous where the laser
diode is being used as a light source as unwanted speckle and interference
effects are greatly reduced.
The most common types on the planet by far are those used in CD players and
CDROM drives. These produce a (mostly) invisible beam in the near infrared
part of the spectrum at a wavelength of 780 nm. The optical power output
from the raw laser diodes may be up to 5 mW but once it passes through the
optics, what hits the CD is typically in the .3 to 1 mW range.
Visible laser diodes have replaced helium neon lasers in supermarket checkout
UPC scanners and other bar code scanners, laser pointers, patient positioning
devices in medicine (i.e., CT and MRI scanners, radiation treatment planning),
and many other applications. The first visible laser diodes emitted at a
wavelength of around 670 nm in the deep red part of the spectrum. More
recently, 650 nm and 635 nm red-orange laser diodes have dropped in price.
Due to the nonuniformity of the human eye's response, light at 635 nm appears
more than 4 times brighter than the same power at 670 nm. Thus, the newest
laser pointers and other devices benefitting from visibility are using these
newer technology devices. Currently, they are substantially more expensive
than those emitting at 670 nm but that will change as DVDs become popular:
Laser diodes in the 635 to 650 nm range will be used in the much hyped DVD
(Digital Video - or Versatile - Disc) technology, destined to replace CDs
and CDROMs in the next few years. The shorter wavelength compared to 780 nm
is one of several improvements that enable DVDs to store about 8 times (or
more - 4 to 5 GB per layer) the amount of information or video/audio as CDs
(650 MB). A side benefit is that dead DVD players and DVDROM drives (I cannot
wait) will yield very nice visible laser diodes for the experimenter :-).
HOW DO I USE A VISIBLE LASER DIODE?
The quick answer is *very carefully* for two reasons:
I am assuming this is a typical 3 to 5 mW visible laser diode probably
emitting at a wavelength in the 635 to 670 nm range.
1. You can easily destroy the typical laser diode through instantaneous
overcurrent, static discharge, probing them with a VOM, or just looking
at them the wrong way :-).
By far the easiest way to experiment with these devices is to obtain
complete laser diode modules. Versions are available with both the drive
circuitry and (adjustable) collimating optics. They are more expensive
than raw laser diodes but are also virtually foolproof. Inexpensive laser
pointers are one source for similar devices which may be adequate for your
needs but modifying them is probably difficult. See the chapter: "Parts
Sources" for suppliers of both raw laser diodes and laser diode modules.
2. Any time you are working with laser light you need to be careful with
respect to exposure of a beam to your eyes. This is particularly true
if you collimate the beam as this will result in the lens of your eye
bringing it to a sharp focus with possible instantaneous retinal damage.
Typical currents are in the 30-100 mA range at 1.7-2.5 V. However, the power
curve is extremely non-linear. There is a lasing threshold below which there
will be no coherent output (though there may be LED type emission). For a
diode rated at a typical current of 85 mA, the threshold current may be 75 mA.
That 10 mA range is all you have to play with. Go to 86 mA (in this example)
and your laser diode may be history in the blink of an eye.
This is one reason why most applications of laser diodes include optical
sensing to regulate beam power. As the temperature of the laser diode changes
(heats with use), the current requirements change as well.
The third lead is for an optical sensing photodiode used to regulate power
output when used in a feedback circuit which controls your current. This
is very important to achieve any sort of stable long term operation.
You can easily destroy a laser diode by exceeding the safe current even for
an instant. It is critical to the life of the laser diode that under no
circumstances do you exceed the safe current limit even for a microsecond!
Laser diodes are also extremely static sensitive, so take appropriate
precautions when handling and soldering. Also, do not try to test them with
an analog VOM which could on the low ohms scale supply too much current.
It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed or have a laser power meter at your disposal,
you can easily exceed the ratings before you realize it.
You might hear someone bragging "I have driven thousands of laser diodes by
just connecting them to a battery and resistor and never have blown any".
Sure, right. While it is quite possible that the susceptibility to instant
damage due to overcurrent varies with the type of laser diode, unless you know
the precise behavior, you must err on the side of caution. Some designers
have gone to extremes, however. See the section: "Laser diode power supply 2"
for a design with 5 levels of protection!
For testing, see the section: "Testing of low power laser diodes".
For an actual application, you should use the optical feedback to regulate
beam power. You should also use a heatsink if you do not already have the
laser diode mounted on one. See the chapter: "Laser Diode Power Supplies".
The raw beam from a laser diode is generally wedge shaped - 10 x 30 degrees is
a typical divergence. You will need a short focal length convex lens to produce
anything approaching a collimated beam. The optics from a dead CD player (even
though CD players and CDROM drives use infra-red laser diodes, the optics
can likely still be used with visible laser diodes), a low to medium power
microscope objective, or even an old disc camera can provide a lens that may
be entirely suitable for your needs.
CD PLAYER LASER DIODES
The major difference between these and the visible laser diodes discussed
in the section: "How do I use a visible laser diode?" is that the output is
near IR - usually at 780 nm (wavelengths from 400 to 700 nm are generally
considered the visible portion of the electromagnetic spectrum). Therefore,
you must use an IR detector device to even confirm laser emission.
Thus, they make truly lousy laser pointers or laser light shows as the emission
is just barely visible in subdued light. If you hoped for a Star Wars type
laser beam, better go hunting for a 25 W argon laser :-). However, for data or
voice communications, various kinds of scanning or sensing, and electro-optic
applications where visibility is not needed or not desirable, these low cost
sources of coherent light are ideal.
Similar types are found in CDROM drives and CD-R recorders, Minidisc equipment,
newer laserdisc players, magneto-optical drives. Other optical storage
technology uses laser diodes as well. WORM drives, in particular, may use
devices with higher power output - 30 mW or more. High resolution laser
imagers, typesetters, and plotters may use laser diodes producing 150 mW or
more. Take additional precautions if you have a laser diode from one of these
(or don't really know where yours spent its earlier life). There are laser
diodes with optical output measured in watts, though these will not be what
you would call tiny and probably require buss bars for electrical power and
plumbing for cooling!
CD laser diodes are infrared (IR) emitters, usually 780 nm, with a maximum
power output of around 5 mW. There is also a very slightly visible deep red
emission from all those I have seen. This may be a spurious very low power
line in the red part of the spectrum or your eye's response to the near IR
appearing red and about 10,000 times weaker than the actual beam. Despite
what the EM spectrum charts show, the eye's response does not drop off to
zero at exactly 700 nm so there decreasing sensitivity out to 800 nm or beyond
depending on the individual. The main beam is IR and invisible. Take care.
A collimated 5 mW beam is potentially hazardous to your eyes. Don't be misled
into thinking the laser is weak due to the weak appearance of the beam. It is
not supposed to be visible at all!
Typical CD laser optics put out about .3-1 mW at the objective lens though
the diodes themselves may be capable of up to 4 or 5 mW depending on type.
If you saved the optical components, these may be useful in generating a
collimated or focused beam. The aspheric objective lens will be optimized
for producing a diffraction limited spot about 1 to 3 mm from its front
surface when the optical system is used intact.
The optics may include a collimating lens, diffraction grating (to produce the
three beams in a three beam pickup), beam splitter prism or mirror, turning
mirror (for horizontally mounted optics), and focusing (objective) lens.
Older pickups tend to have larger and more substantial sets of optics. Despite
their small size and low cost, these are very high quality optical components.
However, depending on design, some of the parts may be missing or combined
into one component. For example, many Sony pickups do not appear to use a
collimating lens. For pickups with a collimating lens, if the objective
lens is removed, you should get a more or less parallel main beam and two
weaker side beams. Mix and match optics for your needs (if you can get it
apart non-destructively). Where there is no collimating lens, the objective
lens may be used for this purpose if positioned closer to the laser diode.
WARNING: A collimated 5 mW beam is hazardous especially since it is mostly
invisible. By the time you realize you have a problem it will be too late.
The coils around the pickup are used for servo control of focus and tracking
by positioning the objective lens to within less than a um (1/25,400 of an
inch) of optimal based on the return beam reflected from the CD. See the
document: "Notes on the Troubleshooting and Repair of Compact Disc Players
and CDROM Drives" for more information on optical pickup organization and
operation.
Typical drive currents are in the 30 to 100 mA range at 1.7 to 2.5 V. However,
the power curve is quite non-linear (though perhaps not as extreme as the
typical visible laser diode). There is a lasing threshold below which there
will be no coherent output (just IR LED emission). For a diode rated at a
nominal current of 50 mA (typical for Sony pickups, for example), the threshold
current may be 30 mA. This is one reason why most applications of laser diodes
include optical sensing (there is a built in photodiode in the same case as
the laser emitter) to regulate beam power. You can easily destroy a laser
diode by exceeding the safe current even for an instant. It is critical to the
life of the laser diode that under no circumstances do you exceed the safe
current limit even for a microsecond!
Laser diodes are also supposed to be extremely static sensitive, so use
appropriate precautions. Also, do not try to test them with an analog VOM
which in particular could on the low ohms scale supply too much current.
It is possible to drive laser diodes with a DC supply and resistor, but unless
you know the precise value needed, you can easily exceed the ratings.
For testing, see the section: "Testing of low power laser diodes".
For an actual application, you should use the optical feedback to regulate
beam power. You should also use a heatsink if you do not already have the
laser diode mounted on one. CD laser diodes are designed for continuous
operation. See the chapter: "Laser Diode Power Supplies".
TESTING OF LOW POWER LASER DIODES
If you have pinouts and specifications for your laser diode, these procedures
can be greatly simplified. The following assumes you know nothing about your
device other than that it is a 3 to 5 mW laser diode.
The first step is to identify which pair of terminals are the laser diode and
photodiode. Your laser diode assembly will be configured like one of the
following:
LD LD LD LD
+--|>|--o A +--|>|--o A +--|<|--o A +--|<|--o A
| | | |
C o--+ C o--+ C o--+ C o--+
| PD | PD | PD | PD
+--|>|--o B +--|<|--o B +--|>|--o B +--|<|--o B
If you are leaving the photodiode installed in the optical block, also see the
section: "Reasons to leave the CD laser diode in the optical block" for
sample connections.
The photodiode's forward voltage drop will be in the approximately .7 V range
compared to 1.7-2.5 V for the laser diode. So, for the test below if you get a
forward voltage drop of under a volt, you are on the photodiode leads. If your
voltage goes above 3 V, you have the polarity backwards. Warning: Some laser
diodes have very low reverse voltage ratings and will be destroyed by modest
reverse voltage. Check your spec sheet. However, the laser diodes found inn
CD players seem to be happy with 4 or 5 volts applied in reverse. Of course,
a shorted or open reading could indicate a defective laser diode or photodiode.
The metal case is often one of the terminals, probably C but not always.
If the laser diode is still connected to its circuitry (probably a printed flex
cable), it is likely that the laser diode will have a small capacitor directly
across its terminals and the optical sensing photodiode will be connected to a
resistor or potentiometer. In particular, this is true of Sony pickups and
may help to identify the correct hookup.
Either of the circuits below can be used to identify the proper connections and
polarity and then to drive the laser diode for testing purposes.
* One approach that works for testing is to use a 0 to 10 VDC supply with a
current limiting resistor in series with the diode:
R1 100 ohms 1 W
+ o--------/\/\--------+-----------+--------+
| | |
Power supply C2 + _|_ C2 _|_ __|__ LD1
0 to 10 VDC 10 uF --- .01 uF --- _\_/_ Laser diode
(No overshoot!) - | | |
| | |
- o--------------------+-----------+--------+
If your power supply has a current limiter, set it at 50 or 60 mA to start.
You can always increase it later.
* Alternatively, a fixed supply with a potentiometer can be used:
R2 100 1 W
+ o-----------+ +----/\/\------+-----------+--------+
| | | | |
10 VDC / ^ | C1 +_|_ C2 _|_ __|__ LD1
Power supply \<----+ R1 10 uF --- .01 uF --- _\_/_ Laser diode
(No overshoot!) / 100 ohms - | | |
| 2 W | | |
- o-----------+--------------------+-----------+--------+
R2 limits the maximum current. If you know the specs for your diode, this
is a good idea (and to protect your power supply as well). You can always
reduce its value if your laser diode requires more than about 85 mA (with
R2 = 100 ohms).
The two capacitors provide some filtering to reduce the risk of a transient
blowing the laser diode. C2 should be mounted close to the laser diode.
Before attempting to obtain lasing action with either of these circuits,
monitor the voltage across what you think is the laser diode as you slowly
increase the power supply or potentiometer.
* If you guessed correctly (or have the pinout diagram from the spec sheet
or determined from its former life), the voltage will increase until around
1.5 to 2 V and then climb more slowly. Don't push your luck unless you are
also monitoring the laser diode current and optical output.
* If you are across the laser diode or photodiode in the reverse biased
direction, the voltage will continue to climb above 2 V without slowing.
Don't push your luck here - the breakdown voltage of the laser diode may
be only a little more than this and - you guessed it - exceeding this is
not healthy for the laser diode either.
* If you are on the photodiode in the forward direction, the voltage will get
stuck around .7 V.
Once you have identified the correct connections, monitor the current through
the laser diode as you check for a laser beam.
* For IR laser diodes, you *must* use an IR detector circuit, card, video
camera or camcorder (with the requisite 3 hands) to monitor for an actual
IR laser beam.
Note: If you are trying to use a video camera or camcorder as an IR detector,
confirm its sensitivity to near IR by looking at an active IR remote control
through its viewfinder. It may have a built in IR blocking filter which will
prevent it from being sensitive to IR. This may be removable.
* For visible laser diodes, you can use your eyeballs or any more sophisticated
detector as desired. Look from an oblique angle or better yet, place a white
card a couple of inches in front of the laser diode. Even a 1 mW laser diode
is an intense source of light - there will be no doubt when lasing begins.
Some typical operating currents for laser diodes of various wavelengths are
listed below. THESE ARE JUST EXAMPLES. Your laser diode may have a lower
operating current than the ones listed here! The lasing threshold may be as
little as 5 or 10 mA below the operating current and the operating current may
be 5 mA or less below the maximum current.
Wavelength Operating Current
808 nm 60 - 70 mA
780 nm 45 - 55 mA
670 nm 30 - 35 mA
660 nm 55 - 65 mA
650 nm 65 - 85 mA
640 nm 70 - 90 mA
Of course, if you inherited a bag of identical laser diodes and can afford to
blow one: (1) I could use a few before you do this :-) and (2) you probably
could fairly accurately characterize them by testing one to destruction.
For a current below the lasing threshold for your laser diode, there will be
some emission due to simple LED action. As you slowly increase the current,
at some point (if the laser diode is good) as you exceed the threshold current,
the character of the emission will change dramatically and a very slight
increase laser diode in current will will result in a significant increase in
intensity. Congratulations! The laser diode is lasing.
Caution: unless you have a laser power meter, don't push your luck. The
maximum safe current may be as little as 5% above the lasing threshold. Go
over by 6% and your diode may be history. The exponential power curve seems
to be steeper with visible laser diodes but there is no way to be sure without
specifications. It is all too easy to convert laser diodes into extremely
useless DELDs - Dark Emitting Laser Diodes - or very expensive LEDs.
I have used this approach with laser diodes from dead CD players without
difficulty. In the case of many of these, the operating current is printed
on a sticker on the optical block, often as a 3 digit number representing
the current in 10ths of mAs. Typical values are 35 to 60 mA (350 to 600).
Sony pickups typically average around 50 mA. Without this information, the
best you can do is to estimate when it is lasing at the proper intensity by
comparing the brightness of the 'red dot' one sees by looking into the lens
from a safe distance at an oblique angle. However, this is not very reliable
as the optical power at the objective lens depends on the particular CD player.
REASONS TO LEAVE THE CD LASER DIODE IN THE OPTICAL BLOCK
There are several good reasons to leave your CD laser diode installed in the
optical block assembly even if you are not going to use it with the objective
lens and focus and tracking actuators:
1. The pickup block provides the very important heat sink which is necessary
for continuous operation.
2. There is less risk of damaging it through careless handling and ESD.
3. There may be a collimator lens in there - probably the first or second
optical element in front of the laser diode. It may be combined with the
laser diode in its metal barrel. If there is a collimator, you should be
able to get a nice nearly parallel beam without much work. At most, a
small lens will be needed to optimize it.
Remove the objective (front) lens and its associated coils unless you
require them for a short range application. They will likely come off as
a unit without too much effort. However, try not to destroy this assembly
as you never can tell what might be needed in the future.
4. The multisegment photodiode sensor and focus and tracking actuators may be
useful for a variety of applications.
While there are many variations on the construction of optical pickups even
from the same manufacturer, they all need to perform the same functions so the
internal components are usually quite similar.
Here is the connection diagram for a typical Sony pickup:
_
R1 +---|<|----o A | +----o F+
+-/\/\---o VR | PDA | (
PD1 | | +---|<|----o B | ( Focus
+---|<|--+---+----o PD (sense) | PDB > Focus/ ( coil
| +---|<|----o C | data (
| LD1 | PDC | +----o F-
+---|<|--+--------o LD (drive) +---|<|----o D _|
| _|_ | PDD _ +----o T+
| --- C1 +---|<|----o E | (
| | | PDE > Tracking ( Tracking
+--------+--------o G (common) +---|<|----o F _| ( coil
| PDF (
Laser diode assembly | +----o T-
+----------o K (Bias+)
(includes LD/PD and Focus/tracking
flex cable with C, R). Photodiode chip actuators
The laser diode assembly and photodiode chip connections are typically all on
a single flex cable with 10 to 12 conductors. The actuator connections may
also be included or on a separate 4 conductor flex cable. The signals may
be identified on the circuit board to which they attach with designations
similar to those shown above. The signals A,C and B,D are usually shorted
together near the connector as they are always used in pairs. The laser
current test point, if present, will be near the connections for the laser
diode assembly.
It is usually possible to identify most of these connections with a strong
light and magnifying glass - an patience - by tracing back from the components
on the optical block. The locations of the laser diode assembly and photodiode
array chip are usually easily identified. Some regulation and/or protection
components may also be present.
Note: There are often a pair of solder pads on two adjacent traces. These
can be shorted with a glob of solder (use a grounded soldering iron!) which
will protect the laser diode from ESD or other damage during handling and
testing. This added precaution probably isn't needed but will not hurt. If
these pads are shorted, then there is little risk of damaging the laser diode
and a multimeter (but do not use a VOM on the X1 ohms range if it has one) can
be safely used to identify component connections and polarity.
See the document: "Notes on the Troubleshooting and Repair of Compact Disc
Players and CDROM Drives" for additional information on construction and
testing of optical pickup assemblies.
LASER DIODE LIFE
For all intents and purposes, laser diodes in properly designed circuits do
not degrade significantly during use or when powered on or off. However, it
doesn't take much to blow them (see the sections: "How do I use a visible
laser diode?" and "CD player laser diodes"). I have seen CD players go more
than 10,000 hours with no noticeable change in performance. This doesn't
necessarily mean that the laser diode itself isn't gradually degrading in some
way - just that the automatic power control is still able to compensate fully.
However, this is a lower bound on possible laser diode life span.
Some datasheets list expected lifetimes for laser diodes exceeding 100,000
hours - over 12 years of continuous operation. Of course, I trust these
about as much as the latest disk drive MTBFs of 1 million hours :-).
Laser diodes that fail prematurely were either defective to begin with or,
their driver circuitry was inadequate, or they experience some 'event'
resultling in momentary (greater than a few microseconds) overcurrent.
As noted elsewhere, a weak laser diode is well down on the list of likely
causes for CD player problems.
Of course, in the grand scheme of things, even LEDs gradually lose brightness
with use.
HOW SENSITIVE ARE LASER DIODES, REALLY?
Not all laser diodes are created equal and their susceptibility to damage
through improper handling or improper drive likely varies widely. Here is
a discussion of some of the issues:
From: Eric Rechner (erechner@jetstream.net)).
"Does anyone have any experience with Hitachi laser diode HL7843MG 5 mW 780nm?
I find this diode to be possibly extremely sensitive (ESD??), more so than
any other 780nm laser diode. Does anyone know if there are problems with
Hitachi MQW type diodes? Are MQW diodes more sensitive to ESD than Double
Heterojunction diodes? Does anyone have info on possibly 'bad' or defective
lasers out there?"
(From: Jon Elson (jmelson@artsci.wustl.edu)).
Strange. I think I've used some of these.
I hear everybody babbling about extreme static sensitivity on these devices,
yet I've never had a failure, and I've been using just the usual minimum
precautions with any semiconductor device. I suspect that people may be
exceeding the optical power MAXIMUMS on the devices. I've been very
conservative on that, since the devices only carry an optical maximum, and
don't have that correlated to forward diode current (difficult, because it
varies strongly with temperature). I try to run them at a good bit less than
rated power, maybe 2-3 mW optical output. I'm using a diode sold by Digi-Key
for $19.00, just because it is cheaper than the Panasonic in the 5.4 mm case.
I think the manufacturer is NVG or something like that. I've got 10 of them I
am working with, designing a closed-loop driver for a photoplotter, which
pulses the lasers on and off as fast as 10 uS on, 10 uS off. It is working
pretty well now. I included a series resistor (as well as the control
transistor), so that if the loop becomes unstable or the sensing diode gets
disconnected, it won't fry the laser diode.
(From: Dr. Mark W. Lund (lundm@xray.byu.edu)).
The babbling starts here: You don't have to be a total idiot to blow these
things, in fact I have blown a few myself. Identifying the source of the
trouble is extremely costly and difficult because it only takes a spike of a
few nS to to the damage. I would say that 99.9999% of the time it is the
power supply. Either it spikes on turn-on, turn-off, or at random. We used
to toast lasers with a $5,000 laser diode power supply that would spike every
time you sent certain signals on the IEEE 488 control line. This was a tough
one to figure out, I can tell you. In the process we tried to damage one
using static to try to get a handle on the sensitivity, but were not able to
get a catastrophic failure this way (we may have induced some latent failures,
however). Other laser diodes may vary.
(From: Jon Elson (jmelson@artsci.wustl.edu)).
Ah! This is good anecdotal evidence! I've often suspected that there might
be more of this going on, and instead of examining the drivers, people just
attribute problems to an invisible gremlin! I sure can see how a closed
circuit driver can oscillate or overshoot on transients, and there could be a
situation where some percentage of drivers will be less stable due to
component tolerances. Unless you rigorously test a good batch of your
drivers, you could have this sort of thing and not know it. (Of course, any
time you put a computer in the loop, especially one that is canned inside
an instrument, then the probability of unanticipated gremlins increases
dramatically!).
Of course, I was designing a fixed-purpose driver to be used in a specific
application, inside an instrument, so I had it easier than the guys designing
a lab-quality pulser for who knows what application. So, I could put in a
resistor, which will limit current to some 'safe' level, even if the loop is
unstable, which it certainly was when I was tuning up my driver.
I DO use generally sound anti-static precautions, almost subconsciously, to
protect all semiconductor devices. But, I am aware that I have occasionally,
by accident, touched a cable going to the laser diode before I was grounded,
and I have never noted a catastrophic failure.
I will have to go through some rigorous life-testing to make sure I'm not
causing latent failures, but I've run these diodes for quite a few hours while
testing things, and nothing of note has turned up yet.
By babbling, I meant some items in print media, as well as a lot on this and
other newsgroups, indicating that if you even touch one lead of a diode laser,
it is ABSOLUTELY destroyed, with a probability of 1.000! Obviously not true!
Your comments are well reasoned, and indicate real experience. Others have
also written that only a huge corporation, with millions in test equipment,
could ever make their own laser diode driver. Now, clearly, the nanosecond
multi-watt pulsers ARE much more difficult to do right, fast risetimes without
overshoot is tricky. But, I did it in my basement with just over $1,000 in
test equipment, mostly a decent oscilloscope. I also had the confidence that
if I DID blow a few diodes, it wasn't so painful at $19 each.
So, now, I'm babbling!
IR DETECTOR CIRCUIT
This IR Detector may be used for testing of IR remote controls, CD player
laser diodes, and other low level near IR emitters.
Component values are not critical. Purchase photodiode sensitive to near IR
(750-900 um) or salvage from opto-coupler or photosensor. Dead computer
mice, not the furry kind, usually contain IR sensitive photodiodes. For
convenience, use a 9V battery for power. Even a weak one will work fine.
Construct so that LED does not illuminate the photodiode!
The detected signal may be monitored across the transistor with an
oscilloscope.
Vcc (+9 V) o-------+---------+
| |
| \
/ / R3
\ R1 \ 500
/ 3.3K /
\ __|__
| _\_/_ LED1 Visible LED
__|__ |
IR ----< _/_\_ PD1 +--------o Scope monitor point
Sensor | |
Photodiode | B |/ C
+-------| Q1 2N3904
| |\ E
\ |
/ R2 +--------o GND
\ 27K |
/ |
| |
GND o--------+---------+
_|_
-
DIVERGENCE OF LASER DIODES
(Portions from: Mark W. Lund (lundm@physc1.byu.edu)).
The divergence specification for laser diodes is measured to the half power
points. T full width at the 10% level may be more like 70 or 80 degrees than
the 30 degrees in the specifications.
A simple short focal length lens will collimate the beam. However, laser
diodes tend to be astigmatic which means that you will have one axis
collimated at a different focus than the other. A typical value for this
astigmatism is 40 microns. A cylindrical lens in addition to the spherical
collimating lens or a special lens designed for this purpose can correct this
but may not be needed for non-critical applications.
Any camera lens will be able to produce a reasonably well collimated beam
(subject to the astigmatism mentioned above). Put the laser diode it at the
focal point of the lens. If you want the type of narrow beam produced by a
HeNe laser, you need a short focal length lens, such as a microscope
objective. A good compromise between cheap and short focal length would be
an old disk camera lens. These cameras can be found at thrift shops, garage
or yard sales, and flea markets for a couple dollars or less.
The longer the focal length the larger your beam will be, but the less effect
the astigmatism will have. The diameter of the beam will be the size of the
aperture of the lens (in which case you are throwing away light) or the size
of the beam at the distance of one focal length, whichever is less.
WHY CAN'T AN LED BE FOCUSED LIKE A LASER DIODE?
The cheap laser diode from a CD player can be focused to a spot less than
2 um in diameter. Why is this not possible with an LED?
The quick answer is that an LED does not appear as a point source and has
as effective emitting area which is huge compared to a laser diode. Even
though the emitting area of a laser diode is not a point, due to the way the
laser beam is generated - collimation wise - it appears as a point source.
And, a point source can be focused to another point.
The effective emitting area of an LED is perhaps .25 x .25 mm. To focus
an incoherent source like this to a 2 um spot with imaging optics would
require a ratio of distances of roughly 125:1 for the LED-to-lens compared
to the lens-to-image plane.
With any kind of real world optics, you will get a vanishingly small amount
of power at the image plane. Similarly, an LED beam cannot be cleaned up
with a spatial filter (pinhole) as very little of the beam will make it
through.
The laser diode is coherent and monochromatic (enough) that relatively simple
optics can be used to focus it to a spot smaller than 2 um. While the
dimensions of the laser diode chip are not all that much different from the
LED, the characteristics of the laser emission makes such focusing a
relatively easy task.
Consider that the beam from a HeNe or ruby laser doesn't come from point
source either. The beam can be sharply focussed because it is very well
collimated.
The availability of relatively cheap laser diodes really was the enabling
technology for the CD revolution.
(From: Steve Nosko (q10706@email.mot.com)).
If a beam of light has nothing but *precisely* parallel rays, it can be
focused to a point. Also, if the beam originated from a point, a lens will
focus it to a point.
An LED has neither of these. First, it is an area source and light coming
from that surface is not parallel. It would also be called a diffuse source,
meaning light from all places on the surface travels in many directions. This
kind of source can not be focused to anything but a smaller image of itself.
The shorter the focal length of the lens, the smaller the image - but it is
still an image of the source, not a spot. It is because of these rays,
traveling in different directions, that a lens can't focus them all to the
same point. If you draw the side view of a lens and trace rays this all
should be obvious.
The gas laser, on the other hand, has rays which are much much closer to being
parallel. The diode laser has rays which appear to come from an apparent point
inside the diode.
There are two more subtle effects. One effect is the relatively wide range of
wavelengths in the LED versus the narrow range of a laser. Simple optics don't
focus all wavelengths at the same focal length. So the wide bandwidth of the
LED causes a little trouble. There is another effect having to do with the
size of the lens (diffraction limit) and the wavelength, but this is also
secondary to an understanding of the *primary* reason why an LED can't be
focused. I'll only talk about the largest effect due to the extended, non
collimated source.
One thing to note is that the laser diode actually has two apparent point
sources. One for the wide axis of the beam and another for the narrow axis.
This means that the lens must be more like two crossed cylindrical lenses with
different focal lengths. There are different types of laser diodes with
varying degrees of this so that some are easier to to design lenses for.
There probably are types, by now, where there aren't two.
I think of it like this (right or wrong). The astigmatism has two components.
One is the difference in divergence between the two axes. I think this can be
even if there is ONLY one apparent point source. It is just a point source
with an oval aperture letting light through. The other is the different
apparent point sources for the two axes.
COMMENTS ON DRIVING LASER DIODES WITHOUT OPTICAL FEEDBACK
(From: Dwight Elvey (elvey@civic.hal.com)).
If you intend to use the laser without the feedback, one has to realize that
there are a number of problems. One is that as the temperature goes down, the
laser efficiency goes up. This tends to cause the laser diode to destroy itself
at lower temperatures while running that same current that was OK at some
higher temperature. Generally, if the temperature doesn't vary to much, one
can use something as simple as a limiting resistor and not run the laser at its
highest output. I once made a burn-in driver for some power lasers that used
constant current sources that had no feed back but I had to preheat the diodes
to 100 degrees C before using that high a level of current. The level of
current used would have wiped the diodes out at room temperatures.
The hardest part of the whole thing was making the circuit to have controlled
levels of current during power on and power off. Most things like op-amps are
not specified under these conditions. My first attempt wiped out 10 diodes :-(
when I turned the power on.
To run the diodes at there maximum light out safely, requires using the feed
back photo diode.
VISIBILITY OF NEAR-IR (NIR) LASER DIODES
The following describes an interesting and convincing experiment. I would
tend to believe these results concluding that the visible light from a CD
laser diode is probably a spurious emission rather than the human eye's
weak sensitivity to 780 nm radiation. The fact that the red emission was
undiminished even after the laser diodes were damaged by overcurrent is
further confirmation of these conclusions. If the red is a spurious
(LED-like) emission, it should appear below the laser threshold suggesting
another test.
(From: Kjell Kraakenes (kkraaken@telepost.no)).
I once used 780 nm laser diodes similar to the types used in CD players, and
something that puzzled me was that I was able to see some red radiation from
these diodes. I used a microscope objective to focus the light on a wall a
few meters away, and when properly focused, a red spot was visible to the
naked eye. I had a piece of black card board on the wall, and there was no
specular reflection. I used an IR viewer of the type sold by Edmund
Scientific (Find-R-Scope), and if I looked at the spot with this IR viewer
the beam appeared defocused. By adjusting the distance between the laser diode
and the microscope objective, the spot (as it appeared through the IR viewer)
could be brought to a better focus. The red, visible light was then so much
defocused that it was no longer visible to the naked eye. From these
observations, I assumed that the spot I saw through the IR viewer was the
laser emission at 780 nm, and that the visible light was some weak emission
at a shorter wavelength. Because of the chromatic aberrations in the
microscope objective these two wavelength could not be expected to be in focus
simultaneously. I did not notice whether the distance between the laser diode
and the microscope objective was increased or decreased when shifting between
the focus of the visible and the IR light, but since I did not know the
chromatic aberrations of the microscope objective this information would not
help me.
I damaged a few of these laser diodes. Probably by burning one of the facets
such that the lasing threshold was increased. Electrically they were OK, and
the visible output appeared as intense as before, but the total output was
only a few microwatts.
I therefore believe that the light people see from NIR laser diodes is
spurious emission within the visible band, and not intense NIR radiation.
(From: Don Klipstein (Don@Misty.com)).
Some nominally IR wavelengths are indeed very slightly visible. In favorable
conditions (mainly isolating from more visible wavelengths) I have seen with
my own eyes:
1. The 766.49/769.9 nM potassium lines, as a contaminant in high pressure
sodium lamps.
2. The 818.3/819.5 nM sodium lines in the spectra of high pressure sodium
lamps.
3. The 762.1, 759.4, and 822.85 nM earth atmospheric absorption lines in the
solar spectrum. (Usually with the sun somewhat low.)
4. The output of a laser diode in my CD player is visible at eye-safe
intensities (half a meter from a source with a beam covering nearly a
steradian for a few seconds). I have seen the spectrum of this along with
that of a neon lamp placed next to it, and verified that what I saw was
the laser line, with a wavelength around 800 nM. It could be as low as
around 780 nM.
According to the C.I.E. "Y" or visibility function (or extrapolation thereof),
the visibility of these lines is impressively low. However, considering the
wide dynamic range of the human eye, these wavelengths are visible at eye-safe
levels.
CAUTION: there is no advance warning of having exceeded eye-safe exposure to
slightly visible wavelengths normally considered IR. You may permanently toast
part of your retinas duplicating the above unless you verify retinal exposure
below the Class I laser exposure limit.
I recently got a laser pointer with a wavelength of 660-661 nm or so and
(guesstimated) 2 mW of output power.
I discovered that if I shine the beam through one of those dielectric
interference bandpass filters, I got some weak beam output at other
wavelengths. So, I investigated further.
About (very roughly estimated from standard issue eyeballs) .2 percent of the
beam is spurious radiation with a continuous spectrum. I don't yet know well
what it does at longer wavelengths, but a majority of the short wavelength
side of this is in the few tens of nm below 660 nm. Slight traces exist down
to 540 nm. With two 532 nm filters, I could stare into the beam and see a dim
point of light. With a 570 nm filter, it was slightly bright to stare into
and I could see the beam VERY DIMLY on a wall in a dark room. With a filter
around 630 nm, I could easily see the beam on a wall in a dark room. I used
my diffraction grating to verify that most of this was continuous spectrum in
the passband of the filter.
The spurious radiation takes the same path that the laser radiation does.
With no filter, I could not see any continuous spectrum with my diffraction
grating. The laser line was so much stronger.
As for IR lasers? If the spectrum is just a long-shifted version of what my
visible laser does, the most visible part of the laser output would be the
laser line. Having a wavelength 100 nm closer to visible increases its
visibility only by about a factor of 1,000 and the total spurious output was
(roughly) 1/1,000 of the laser line output. The wavelength of the bulk of
this was nowhere near 100 nm shorter.
Although I can't be sure this would always be the case, the only spectrum
components I could see using a diffraction grating with my CD player
laser was the laser line at about 800 nm.
I suspect different IR laser diodes may have greatly different ratios of laser
and LED output. If the LED output is only a fraction of a percent of the laser
output, the visible output would be mainly the slightly visible laser line. If
the LED output is equal to a few percent or more of the laser output, then it
may be more visible than the laser line.
WHEN WILL WE SEE GREEN AND BLUE DIODE LASERS?
(Portions from: Adam Cohen (adc20@eng.cam.ac.uk)).
Blue and green has been widely demonstrated by SHG (second harmonic generation
a.k.a. frequency doubling) in nonlinear crystals (lithium niobate, KTP et
al.), organic nonlinear materials, etc. etc.
The direct emission from a semiconductor has been the Holy Grail for several
years. The semiconductor materials available with a sufficiently wide
band-gap are notoriously difficult to deposit and cleave....But several groups
are close to a commercial device now. In Japan, Nichia Chemicals, Sony,
Pioneer and Toshiba (see p26 of Laser Focus World, March 1997) are all working
on GaN-based devices (active layer in the Toshiba device is actually InGaN). I
think 3M and some other US firms were concentrating on ZnSe, which emits at a
slightly longer wavelength (more blue-green than blue)....
LASER DIODE POWER SUPPLIES
LASER DIODE DRIVE REQUIREMENTS
The following must be achieved to properly drive a laser diode and not ruin
it in short order:
* Absolute current limiting. This includes immunity to power line transients
as well as those that may occur during power-on and power-off cycling. The
parameters of many electronic components like ICs are rarely specified
during periods of changing input power. Special laser diode drive chips
are available which meet these requirements but a common op-amp may not be
suitable without extreme care in circuit design if at all.
* Current regulation. Efficiency and optical power output of a laser diode
goes up with decreasing temperature. This means that without optical
feedback, a laser diode switched on and adjusted at room temperature will
have reduced output once it warms up. Conversely, if the current is set up
after the laser diode has warmed up, it will likely blow out the next time
it is switched on at room temperature.
Note that the damage due to improper drive is not only due to thermal effects
(though overheating is also possible) but due to exceeding the maximum optical
power density at one of the end facets - and thus the nearly instantaneous
nature of the risk.
Many semiconductor manufacturers offer laser driver chips. Some of these
support high bit rate modulation in addition to providing the constant current
optically stabilized power supply. Other types of chips including linear
and switching regulators can be easily adapted to laser diode applications
in many cases:
* Maxim (http://www.maxim-ic.com/).
The MAX3261 (1.2 Gbps) and MAX3263 (155 Mbps) laser driver driver chips
are two examples of their highly integrated solutions.
* Linear Technology (http://www.linear-tech.com/).
App Note AN52 (and probably others) includes a sample circuit using their
one of their chips (not necessary dedicated laser drivers) for powering
laser diodes. In AN52, the LT1110 Micropower DC-DC converter is used as
the current regulator for operating from a 1.5 V battery.
* Both Sharp and Mitsubishi manufacture IC's for driving laser diodes. Most
will maintain constant power. Some require two voltages, others just one.
These circuits will drive the common cathode lasers, or the Sharp "P" or the
Mitsubishi "R" configuration which has the laser's cathode connected the the
anode of the photo diode. The Sharp IR3C07 is a good for CW or analog
modulation, and the IR3C08 or IR3C09 will allow digital modulation to 10
MHz. These parts are quite inexpensive.
* Analog Devices (http://www.analog.com/) has several laser diode drivers
including the AD9660 and AD9661 both of which provide for full current
control using the photodiode for feedback and permit high speed modulation
between two power levels.
* Burr-Brown (http://www.burr-brown.com/):
(From: Steve White (stevew@hitl.washington.edu)).
We are using the OPA 2662 (Burr-Brown) for this. It is an OTA with 370MHz BW,
59mA/ns SR, and can source/sink 75mA of current per channel (two channels per
chip which may be paralleled quite easily). The part provides the emitter of
the current source to an external pin (programming side of an internal
current mirror), so that a single resistor sets the voltage-current transfer
characteristic. Watch out for the dependence of the harmonic distortion specs
upon the supplied current and frequency though...if this will be a problem
for your particular application that is (didn't matter much for mine).
VISIBLE LASER DIODE POWER SUPPLIES (reverse engineered from commercial units)
These circuits were traced from commercial CW laser lights (these were used
for positioning in medical applications). Errors may have been made in the
transcription. The type and specifications for the laser diode assembly (LD
and PD) are unknown. The available output power of both of these lasers is
about 1 mW but the circuits should be suitable for the typical 3 to 5 mW
visible laser diode (assuming the same polarity of LD and PD or with suitable
modifications for different polarity units.)
If you do build these or any other circuits for driving a laser diode, test
them first with a combination of a visible LED and silicon diodes (to simulate
the approximate expected voltage drop) and a discrete photodiode to verify the
current limited operation. Them with the laser diode in place, start with a
low voltage supply until you have determined optimal settings and work up
gradually. Laser Diodes are NOT very forgiving.
LASER DIODE POWER SUPPLY 1
This one runs off a (wall adapter) power supply from about 6 to 9 V.
D1
Vcc o-----|>|-------+------------+-----------------+-----+--------+
1N4001 | | | | |
Rev. Prot. | | Power Adjust | _|_ __|__
| / R3 10K (2) | PD /_\ LD _\_/_
| R2 \ +----+ | | |
| 560 / | | +-----|---||---+
| \ +---/\/\--+-------+ C4 |
| | | | .1 uF |
| | | +----||----+ /
+_|_ | | __|__ C2 (1)| \ R4
C1 --- | | E / \ 100 pF| / 3.9
10 uF - | +-----|------' Q1 '-------+ |
| |R | BC328-25 (5) | |
| +---+ | (PNP) | |/ Q2 (5)
| | _|_. | +---| BD139
| VR1 +-'/_\ | | |\ (NPN)
| LM431 | | C3 +_|_ E|
| 2.5 V | | 10 uF --- |
| (3) | |X - | |
R1 3.9 | | |Y | |
GND o----/\/\/\-----+------------+-----+--------------------+-----+
Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.
Notes:
1. Capacitor C4 value estimated.
2. Potentiometer R3 measured at 6K.
3. LM431 shunt regulator set up as 2.5 V zener.
4. Supply current measured at 150 mA (includes power on LED not shown).
5. Transistor types do not appear to be critical.
LASER DIODE POWER SUPPLY 2
This one, from the same manufacturer as the one described in the section:
"Laser diode power supply 1", seems to be an improved design including a
soft-start (ramp-up) circuit and an inductor in series with the laser diode.
Otherwise, it is virtually identical and runs off of a 6 to 9 V DC source.
Since both units were from the same company, I assume that these refinements
were added as a result of reliability problems with the previous design - in
fact, I have recently discovered that the unit from which I traced that
schematic is not as bright as the one below!
Interestingly, there is no longer any reverse polarity input protection - I
don't know why that would have been removed! C1 and Q1, at least, would
likely let their smoke out if this circuit was hooked up backwards.
2SC517 (NPN) (6)
Vcc o-----+--. Q1 .---+---------+---------------------+-----+--------+----+
| _\___/_E | | | | | |
| | | | | _|_ __|__ \ R5
R1 \ | | | | PD /_\ LD _\_/_ / 1K
3.3K / | | / | | | \
\ | | R2 \ | | | |
| | | 390 / R3 +-----|---||---+----+
| | | \ +---/\/\------+-------+ C4 (2) |
+-----+ | | | 2.2K | 10 pF +
| | | | +----||----+ )
| +_|_ C2 | | __|__ C3 (1) | ) L1
| --- 33 uF | | R4 E / \ 47 pF | ) (3)
| - | +-----|--/\/\----' Q2 '-------+ +
| | |R | 220 BC328-25 (6) | |
C1 +_|_ | +---+ \ (PNP) | |/ Q3 (6)
1 uF --- | | _|_. /<-+ R6 +---| BD139
- | | VR1 +-'/_\ \ | 10K | |\ (NPN)
| | LM431 | | | Power Adjist C5 +_|_ E|
| | 2.5 V | +--+ (4) 10 uF --- |
| | (5) | |X - | |
| | | |Y | |
GND o-----------+------+---------+-----+------------------------+-----+
Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.
Notes:
1. Capacitor C3 was marked n47 and very small. Guessing 47 pF.
2. Capacitor C4 was marked 10n and very small. Guessing 10 pF.
3. Inductor marked Red-Black-Black-Silver ??.
4. Potentiometer R6 setting not measured.
5. LM431 shunt regulator set up as 2.5 V zener.
6. Transistor types do not appear to be critical.
LASER DIODE POWER SUPPLY 3
This one runs off of a (wall adapter) power supply from about 10 to 15 V
(12 V nominal).
It was apparently designed by someone who was totally obsessed with protecting
the laser diode from all outside influences - as one should be but there are
limits :-). This one goes to extremes as there are 5 levels of protection:
1. Input C-L-C filter.
2. Soft start circuit (slow voltage ramp up).
3. LM7810 voltage regulator.
4. LT1054 DC-DC voltage converter.
5. Optical power based current source.
The first part of the circuit consists of the input filter, soft start circuit,
voltage regulator, and DC-DC voltage converter. Its output should be s super
clean, filtered, despiked, regulated, smoothed, massaged source of -10 V ;-).
MPSA13
L1 D1 C E I +--------+ O -10 V out
+12 o--+--CCCC--+--|>|--+--. Q1 .---+--| LM7810 |--+-------+ o
| |1N4002 | _\___/_ | +--------+ | | C5 |
| | R4 / | | C| | | +------+
| | 10K \ | | | | 8| 7 6 5| 180 |
| | / | | | | +-+--+--+--+-+ uF |
+_|_ C10 +_|_ C11 | | +_|_ C8 | C7 _|_ | |16 V|
--- 2.2 --- 2.2 +-----+ --- .22 | .1 --- | LT1054 | +_|_
- | uF - | uF | | - | uF | uF | | | ---
| | +_|_ _|_ | | | +-+--+--+--+-+ - |
| | C9 --- --- C6 | | | 1 2| 3| 4| C3 |
| | 4.7 - | | .047 | +------+-------------+--|--||--+
| L2 | uF | | uF | | C4 |+ - |.01 uF|
Gnd o--+--CCCC--+-------------+------+-------+ 180 uF +-|(--+------+
16 V
It was not possible to determine the values of L1 and L2 other than to measure
their DC resistance - 4.3 ohms. The LT1054 (Linear Technology) is a 'Switched
Capacitor Voltage Converter with Regulator' running at a 25 KHz switching
frequency. A full datasheet is available at http://www.linear-tech.com/.
The input to the LM7810 ramps up with a time constant of about 50 ms (R4
charging C9). This is regulated by the LM7810.
The LT1054 takes the regulated 10 V input and creates a regulated -10 V output.
There is no obvious reason for using this part except the desire to isolate
the laser diode as completely as possible from outside influences. Like the
use of an Uninterruptible Power Source (UPS) to protect computer equipment from
power surges, a DC-DC converter will similarly isolate the laser diode circuit
from any noise or spikes on its input.
The second part of the circuit is virtually identical to that described in
the section: "Laser diode power supply 1":
Gnd o--------+------------+-----------------+-----+--------+
| | | | |
| | Power Adjust | _|_ __|__
| / R2 20K | PD /_\ LD _\_/_
| R1 \ +----+ | | |
| 470 / | | +-----|---||---+
| \ +---/\/\--+-------+ C2 |
| | | | /
+_|_ | | __|__ \ Rx
C1 --- | | E / \ C /
10 uF - | +-----|------' Q1 '-------+ |
| |R | PN2907 | C|
| | \ (PNP) | |/ Q2
| _|_. / R3 +---| PN2222
| VR1 '/_\ \ 1K | |\ (NPN)
| LM385 | / C1 +_|_ E|
| Z2.5 | | 10 uF --- |
| | |X 16 V - | |
| | |Y | |
+V o---------+------------+-----+--------------------+-----+
Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.
I suspect that there are additional components inside the laser diode assembly
itself (like the hypothetical Rx, probably a few ohms) but could not identify
anything since it is totally potted.
LASER DIODE MODULATION
Neither of these designs can be modulated at any reasonable rate without
modifications to reduce the heavy filter capacitance at multiple locations.
However, in principle, this should be straightforward. Since both the
following affect the optical feedback, attempt at your own risk.
A bi-level modulation scheme could be easily implemented by connecting a
general purpose NPN transistor across an additional resistor (at point XY).
Then, full power will be achieved with the transistor turned on and reduced
power with it turned off. Select a value for R2 that will still maintain
the current above the lasing threshold - 1K is just a start.
X
o
|
+-----+
|C | Typical transistors: 2N2222, 2N3904.
R1 |/ /
TTL Input o---/\/\---| Q1 \ R2
1K |\ / 1K
|E |
Y o------------+-----+
Here is another circuit which should achieve somewhat linear control of laser
power since optical power output should be proportional to photodiode current.
Resistor values shown are just a start - you will need to determine these for
your specific laser diode and operating point.
R
o
|
\ R1 X
/ 10K o
\ |
C1 10 uF | |/ C
o------)|----+--------| Q1
- + | |\ E
Line level | 2N3904 |
audio / /
R2 \ R3 \
o 10K / 1K /
| | |
Y o-----------+------------+----------+
SIMPLE LASER DIODE POWER SUPPLY:
(From: Brian Mork (mork@usa.net)).
Best circuit I've found:
In +-------+ Out 18 ohm*
(+) o-----+---| LM317 |-------/\/\/\----+-----+------o LD anode
| +-------+ | |
_|_ | Adjust | _|_ __|__
22 uF --- +---------------------+ --- 1 uF _\_/_
| | |
| |
(-) o-----+-----------------------------------+------o LD cathode
* Note: Resistor value depends on your specific laser diode current
requirements. Discussion below assumes a laser diode with a 72 to
100 mA drive range --- sam.
Power is 5.5 to 9 VDC. I use a 9 volt battery.
Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic
transistor type case) and the LM317T (TO-220 7805-type case), the pins are,
left to right, Adjust-Output-Input.
For the resistor, I use a small carbon 10 ohm in series with a precision
10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms.
On initial power on, use three garden variety diodes stacked in series
instead of the laser diode. Put a current meter in series with the diode
stack and adjust the precision resistor for 50-60 mA. Disconnect power and
replace the diode stack with the laser diode. Connect up power again, still
watching on the current meter. The diode will probably initially glow
dimly. I use a diode that lases at about 72 mA, and has a max rating of 100
mA. I use about 85 mA for normal ops.
Turn up the current, never exceeding your diode's max limit. The dim glow
will increase in intensity, but at some point, a distinctive step in
intensity will occur. Your diode is lasing. Remove the current meter as
desired. Enjoy!
CONSTANT CURRENT SUPPLY FOR HIGH POWER LASER DIODES:
(From: Winfield Hill (hill@rowland.org)).
The schematic in the section: "Laser Diode power supply" is the standard
circuit for making a constant current source from an LM317 or LM338 (e.g. see
The Art of Electronics, fig 6.38). The problem with this circuit is that for
large currents (the only currents for which it has good accuracy, and is a
serious part saver) it's hard to make the current variable.
For example, for a 3.5 A current source, the resistor value is 0.357 ohms,
and if you then want a 3.1 A current you've got to unsolder it and replace
it with a 0.403 ohm resistor. Bummer.
One option would be to put a low value pot across the sense resistor and
connect its tap to the voltage regulator common/adjust terminal. This will
work reasonably well for a modest current range - perhaps up to 2:1 as shown
below - but runs into difficulties where a wide range of control is desired.
In +-------+ Out R1 1.01 ohm
Vin o----| LM317 |---+-----------/\/\----+----o 1.25 to 2.5 A current source
+-------+ | |
| Adj. +---/\/\-----/\/\---+
| R2 ^ R3
| 100 ohms | 100 ohms
+----------------------+
The reason is that this arrangement can only *increase* the current from the
nominal I = 1.25V/R. So, for example, to get a 10:1 range, the voltage across
the sense resistor would be 12.5 V for the 10x current! In general this is not
attractive for the high current condition because not only have you required
a higher supply voltage, at the maximum current, but the power dissipation in
the sense resistor is also quite high (more like HUGE --- sam).
Let me offer the following simple circuit, which I just created and haven't
tried but 'oughta work' as a solution to this problem.
By contrast, this circuit can only *decrease* the current from the 1.25V/R
value, but it easily handles a 10:1 range (or even much more) and the voltage
across the sense resistor is never more than 1.25V, allowing low supply voltage
(e.g. 5 V) and keeping the dissipation low.
In +-------+ Out R1 .25 ohms
Vin o----| LM338 |-------/\/\/----+-----o 0 to 5 A current source
+-------+ |
| Adj. +----+
| cw | |
| 1K ^ / _|_,
+-------------->\ '/_\ LM385-1.2
/ |
| |
+----+
|
+------------------------+
| I = 0.5 to 1.5 mA sink |
+------------------------+
_|_
-
The 1K pot selects a portion of the floating 1.23 V reference voltage, and
tricks the LM317 or LM338 into correspondingly reducing the voltage across
the 0.25 ohm current-sense resistor. The pot is conventional and may be
panel mounted. It should be possible to nearly shut off the LM338 (a
minimum quiescent current will still flow). The current sink, I, which
powers the floating 1.23 V reference, is not critical and may be a simple
current mirror (sorry to see the TL011 gone!), or even a resistor to
ground or any available negative voltage, depending upon the desired
current-source voltage-compliance range. That's it!
HELIUM NEON LASERS
INTRODUCTION
A helium neon (henceforth abbreviated HeNe) laser is basically a fancy neon
sign with mirrors at both ends. Well, not quite, but really not much more
than this. The gas fill is a mixture of helium and neon gas at low pressure.
A pair of mirrors (one totally reflective, the other partially reflective at
the wavelength of the laser's output) complete the resonant cavity. This is
called a Fabry-Perot cavity (if you want to impress your friends). The
mirrors may be internal (common on small and inexpensive tubes) or external.
Electrodes sealed into the tube allow for the passage of high voltage DC
current to excite the discharge.
I remember doing the glasswork for a 3 foot long HeNe laser which included
joining side tubes for the electrodes and exhaust port, fusing the electrodes
themselves to the glass, preparing the main bore (capillary), and cutting the
angled Brewster windows (so that external mirrors could be used) on a diamond
saw. I do not know if the person building the laser ever got it to work but
suspect that he gave up or went on to other projects (which probably were also
never finished).
Some die-hards still construct their own HeNe lasers from scratch. Once all
the glasswork is complete, the tube must be evacuated, baked to drive off
surface impurities, backfilled with a specific mixture of helium to neon at a
pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760
Torr - 760 mm of mercury), and sealed. The mirrors must then be painstakingly
positioned and aligned. Finally, the great moment arrives and the power is
applied. You also constructed your high voltage power supply from scratch,
right? With luck, the laser produces a beam and only final adjustments to the
mirrors are then required. All sorts of things can go wrong. With external
mirrors, the losses may be too great resulting in insufficient optical gain in
the resonant cavity. The gas mixture may be incorrect or become contaminated.
Seals might leak. It just may not be your day! Nonetheless, if you really
want to be able to say you built a laser from the ground up, this is the
approach to take.
However, for most of us, 'building' a HeNe laser is like 'building' a PC:
An inexpensive HeNe tube and power supply are obtained, mounted, and wired
together. Optics are added as needed. Power supplies may be home-built as
an interesting project but few have the desire, facilities, patience, and
determination to construct the actual HeNe tube itself.
The most common sealed HeNe laser tubes are between 6" and 14" (150 mm to 350
mm) in overall length and 1" to 1-1/2" (25 mm to 37.5 mm) generating optical
power from .5 mW to 5 mW.
Slightly smaller tubes (less than .5 mW), somewhat larger tubes (up to 20 mW),
and much larger tubes with internal or external mirrors (a *meter* or more in
length generating up to 250 mW of optical power), are also available and may
turn up on the surplus market. Specialized configurations - a triple XYZ axis
triangular cavity laser in a solid glass block for an optical ring laser gyro,
for example - also exist but are much much less common - you probably won't
find one of these at a local flea market!
Manufacturers include Aerotech, Melles-Griot, Siemens, Spectra-Physics, and
many others.
HeNe lasers used to be found in all kinds of equipment including early laser
printers, laserdisc players, small laser shows, optical surveying and tunnel
boring systems, medical positioning systems, and supermarket checkout UPC and
other barcode scanners. (You can tell if you local ACME supermarket uses
a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm
wavelength beam from a HeNe laser is noticeably more orange than the 670 nm
deep red from a typical laser diode type.)
Nowadays, these applications are likely to use the much more compact lower
(drive) power solid state laser diodes. Thus, a 5 mW laser pointer complete
with batteries can conveniently fit on a keychain and generate the same beam
power as a HeNe laser half a meter long!
So why bother with a HeNe laser at all? There are several reasons:
* For many applications including holography and interferometry, the high
quality stable beam of a HeNe laser is unmatched (at least at reasonable
cost, perhaps at all) by laser diodes. In particular, the coherence length
and monochromicity of even a cheap HeNe laser are excellent and the beam
profile is circular (laser diodes usually have some amount of astigmatism)
so that simple spherical optics can be used for beam manipulation.
* As noted in the chapter on laser diodes, it is all too easy to ruin them in
the blink of an eye (actually, the time it takes light to travel a few feet).
It would not take very long to get frustrated burning out $50 diodes. So,
the HeNe laser tube may be a better way to get started. They are harder to
damage through carelessness or design errors. Just don't get the polarity
reversed or exceed the tube's rated current for too long - or drop them on
the floor! And, take care around the high voltage.
* Laser diode modules at a wavelength of 635 nm may be somewhat more expensive
than surplus HeNe tubes with power supplies. However, with the introduction
of DVD players and DVDROM drives, this situation probably will not last long.
HeNe LASER SAFETY:
As with *any* laser, proper precautions must be taken to avoid any possibility
of damage to vision. The types of HeNe lasers dealt with in this document are
classified as type II, IIIa, or the low end of IIIb (see the section: "Laser
safety classification". For most of these, common sense (don't stare into
the beam) and fairly basic precautions suffice since the reflected or scattered
light will not cause instantaneous injury and is not a fire hazard.
However, unlike those for laser diodes, HeNe power supplies utilize high
voltage (several KV) and some designs may be potentially lethal. This is
particularly true of AC line powered units since the power transformer may
be capable of much more current than is actually required by the HeNe laser
tube - especially if it is home built using the transformer from some other
piece of equipment (like an old tube type console TV or that utility pole
transformer you found along the curb) which may have a much higher current
rating.
The high quality capacitors in a typical power supply will hold enough charge
to wake you up - for quite a while even after the supply has been switched off
and unplugged. Unless significantly oversized, this isn't usually a lethal
amount of energy but can still be quite a jolt. The HeNe tube itself also
acts as a small HV capacitor so even touching it should it become disconnected
from the power supply may give you a tingle. This probably won't hurt you
physically but your ego may be bruised if you then drop the tube and it
shatters on the floor! Use an insulated 1 M, 2 W resistor to drain the charge
before touching anything.
See the document: "Safety Guidelines for High Voltage and/or Line Powered
Equipment" for detailed information before contemplating the inside or HV
terminals of a HeNe power supply!
COMMENTS ON HeNe LASER SAFETY ISSUES
(Portions from: Robert Savas (jondrew@mail.ao.net)).
A 10 mw HeNe laser certainly presents an eye hazard.
According to American National Standard, ANSI Z136.1-1993, table 4 Simplified
Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective
eyewear with an attenuation factor of 10 (Optical Density 1) is required for a
HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to
10 seconds, the time in which they eye would blink or change viewing direction
due the the uncomfortable illumination level of the laser. Eyeware with an
attenuation factor of 10 is roughly comparable to a good pair of sunglasses
(this is NOT intended as a rigorous safety analysis, and I take no
responsibility for anyone foolish enough to stare at a laser beam under any
circumstances). This calculation also assumes the entire 10 milliwatts are
contained in a beam small enough to enter a 7 millimeter aperture (the pupil
of the eye). Beyond a few meters the beam has spread out enough so that only
a small fraction of the total optical power could possible enter the eye.
INSTANT HeNe LASER THEORY
The term laser stands for "Light Amplification by Stimulated Emission of
Radiation". However, lasers as most of us know them, are actually sources of
light - oscillators rather than amplifiers. (Although laser amplifiers do
exist in applications as diverse as fiber optic communications repeaters and
multi-gigawatt laser arrays for inertial fusion research.) Of course, all
oscillators - electronic, mechanical, or optical - are constructed by adding
the proper kind of positive feedback to an amplifier.
All materials exhibit what is known as a bright line spectra when excited in
some way. In the case of gases, this can be an electric current or (RF) radio
frequency field. In the case of solids like ruby, a bright pulse of light
from a xenon flash lamp can be used. The spectral lines are the result of
spontaneous transitions of electrons in the material's atoms from higher to
lower energy levels. A similar set of dark lines result in broad band light
that is passed through the material due to the absorption of energy at specific
wavelengths. Only a discrete set of energy levels and thus a discrete set of
transitions are permitted based on quantum mechanical principles (well beyond
the scope of this document, thankfully!). The entire science of spectroscopy
is based on fact that every material has a unique spectral signature.
The HeNe laser depends on energy level transitions in the neon gas. In the
case of neon, there are dozens if not hundreds of possible wavelength lines of
light in this spectrum. Some of the stronger ones are near the 632.8 nm line
of the common red-orange HeNe laser - but this is not the strongest:
The strongest red line is 640.2 nm. There is one almost as strong at 633.4
nm. That's right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak
in an ordinary neon spectrum, due to the high energy levels in the neon atom
used to produce this line.
There are also many infra-red lines and some in the orange, yellow, and green
regions of the spectrum as well.
The helium does not participate in the laser (light emitting) process but is
used to couple energy from the discharge to the neon through collisions with
the neon atoms. This pumps up the neon to a higher energy state resulting in
a population inversion meaning that more atoms in the higher energy state than
the ground or equilibrium state.
It turns out that the upper level of the transition that produces the 632.8
nm line has an energy level that almost exactly matches the energy level of
helium's lowest excited state. The vibrational coupling between these two
states is highly efficient.
You need the gas mixture to be mostly helium, so that helium atoms can be
excited. The excited helium atoms collide with neon atoms, exciting some of
them to the state that radiates 632.8 nm. Without helium, the neon atoms
would be excited mostly to lower excited states responsible for non-laser
lines.
A neon laser with no helium can be constructed but it is much more difficult
without this means of energy coupling. Therefore, a HeNe laser that has lost
enough of its helium (e.g., due to diffusion through the seals or glass) will
most likely not lase at all since the pumping efficiency will be too low.
There are many possible transitions from the excited state to a lower energy
state that can result in laser action. The most important (from our
perspective) are listed below:
Wavelength Color
543.5 nm Green
593.9 nm Yellow
611.8 nm Orange
632.8 nm Red-Orange
1152.3 nm Near Infra-Red
3391.3 nm Mid Infra-Red
While we normally don't think of a HeNe laser as producing an infra-red (and
invisible) beam, the IR spectral lines are quite strong. In fact, the first
HeNe laser operated at 1152.3 nm. HeNe lasers at all of these wavelengths are
commercially available but those operating at 632.8 nm are by far the most
common and least expensive.
When the HeNe gas mixture is excited, all possible transitions occur at a
steady rate due to spontaneous emission. However, most of the photons are
emitted with a random direction and phase, and only light at one of these
wavelengths is usually desired in the laser beam. At this point, we have
basically the glow of a neon sign with some helium mixed in!
To turn spontaneous emission into the stimulated emission of a laser, a way
of selectively amplifying one of these wavelengths is needed and providing
feedback so that a sustained oscillation can be maintained. This may be
accomplished by locating the discharge between a pair of mirrors forming
what is known as a Fabry-Perot resonator or cavity. One mirror is totally
reflective and the other is partially reflective to allow the beam to escape.
These mirrors are normally made to have peak reflectivity at the desired laser
wavelength. When a spontaneously emitted photon resulting from the transition
corresponding to this peak happens to be emitted in a direction nearly parallel
to the long axis of the tube, it stimulates additional transitions in excited
atoms. These atoms then emit photons at the same wavelength and with the same
direction and phase. The photons bounce back and forth in the resonant cavity
stimulating additional photon emission. Each pass through the discharge
results in amplification - gain - of the light. If the gain due to stimulated
emission exceeds the losses due to imperfect mirrors and other factors, the
intensity builds up and a coherent beam of laser light emerges via the
partially reflecting mirror at one end. With the proper discharge power, the
excitation and emission exactly balance and a maximum strength continuous
stable output beam is produced.
Spontaneously emitted photons that are not parallel to the axis of the tube
will miss the mirrors entirely or will result in stimulated photons that are
reflected only a couple of times before they are lost out the sides of the
tube. Those that occur at the wrong wavelength will be reflected poorly if at
all by the mirrors and any light at these wavelengths will die out as well.
MODES OF OPERATION
The physical dimensions of the Fabry-Perot resonator impose some additional
constraints on the resulting beam characteristics.
While it is commonly believed that the 632.8 (for example) transition is
a sharp peak, it is actually a gaussian - bell shaped - curve. In order for
the cavity to resonate strongly, a standing wave pattern must exist. This
will only occur when an integral number of half wavelengths fit between the
two mirrors. This restricts possible axial or longitudinal modes of
oscillation to:
L * 2 c * n
W = --------- or F = ---------
n L * 2
where:
* L is the distance between the mirrors (m).
* W denotes the possible wavelengths of oscillation (m).
* n is a large integer (order of 948,000 for W around 632.8 nm, L = .3 m).
* F denotes the possible frequencies of oscillation (Hz).
* c is the speed of light (approximately 300 million m/s).
The laser will not operate with just any wavelength - it must satisfy this
equation. Therefore, the output will not usually be a single peak at 632.8 nm
but a series of peaks around 632.8 nm spaced c/(L * 2) Hz apart. Longer
cavities result in closer mode spacing and a larger number of modes since the
gain won't fall off as rapidly as the modes move away from the peak. For
example, a cavity length of 150 mm results in a longitudinal modes spacing of
about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral
lines in the output will be nearest the combined peak of the lasing medium and
mirror reflectivity but many others will still be present. This is called
multimode operation.
Think of the vibrating string of a violin or piano. Being fixed at both ends,
it can only sustain oscillations where an integer number of cycles fits on the
string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5
(harmonics or overtones). Due to the tension and stiffness of the string,
only small integer values for n are present with a significant amplitude. For
a HeNe laser, the distribution of the selected neon spectral line and shape
of the reflectivity function of the mirrors with respect to wavelength
determine which values of n are present. For a typical HeNe laser tube,
possible values of n will form a series of very large numbers like 948,123,
948,124, 948,125, 948,126,.... rather than 1, 2, 3, 4 :-). For example:
I| .
| | | |
| | | | | |
| | | | | | | |
_______|______.__|__|__|__|__|__|__|__|__|__._______
n=948,125 -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
This also means that as the tube warms up and expands, these spectral line
frequencies are going to drift downward (toward longer wavelengths). However,
since the reflectivity distribution of the mirrors remains constant, new lines
will fill in from above so the overall shape of the output doesn't change.
Thus, a typical HeNe laser is not monochromatic though the effective spectral
line width is very narrow compared to common light sources. Additional effort
is needed to produce a truly monochromatic source operating in a single
longitudinal mode. One way to do this is to introduce another adjustable
resonator called an etalon into the cavity. Then, only modes which are the
same in both resonators will produce enough gain to sustain laser output. By
adjusting the etalon, one spectral line can be selected resulting in single
mode operation.
Lasers can also operate in various transverse modes. Laser specifications
will usually refer to the TEM00 mode. This means Transverse Electromagnetic
Mode 0,0 and results in a single beam. The long narrow bore of a typical HeNe
laser forces this mode of oscillation. With a wide bore multiple sub-beams
can emerge from the same cavity in two dimensions. The TEM mode numbers
(TEMxy) denote the number (minus one) of such sub-beams:
O OO OOO Each 'O' represents
O OO O OO OOO is a single sub-beam.
TEM00 TEM10 TEM01 TEM11 TEM21
Other (non-cartesian) patterns of modes may also be possible depending on
tube dimensions and operating conditions.
EARLY VS. MODERN HeNe LASERS
In the first HeNe lasers (see the diagram below), exciting the gas atoms to
the higher energy level was accomplished by coupling a radio frequency (RF)
source (i.e., a radio transmitter) to the tube via external electrodes.
Modern HeNe lasers almost always operate on a DC discharge via internal
electrodes.
Bellows Bellows
/\/\/\ Discharge tube with external electrodes /\/\/\
|| \________________________________________________/ ||
|| | | | | | | ||===> Laser
|| ___ __|_|________________|_|______________|_|__ ||===> Beam
|| / || | | | \ ||
\/\/\/ || | o | \/\/\/
Adjustable || +-----------o RF exciter o----------+ Adjustable
totally || partially
reflecting ||<-- to vacuum system reflecting
mirror mirror
Early HeNe lasers were also quite large and unwieldy in comparison to modern
devices. A laser such as the one depicted above was over 1 meter in length
but could only produce about 1 mW of optical beam power! The associated RF
exciter was as large as a microwave oven. With adjustable mirrors and a
tendency to lose helium via diffusion under the electrodes, they were a
finicky piece of laboratory apparatus with a lifetime measured in hundreds
of operating hours.
In comparison, a modern 1 mW sealed HeNe laser tube can be less than 150 mm
(6 inches) in total length, may be powered by a solid state inverter the size
of a stick of butter, and will last more than 20,000 hours without maintenance
of any type or a noticeable change in its performance characteristics.
HeNe LASER OPERATION:
The following applies to most of the inexpensive sealed low to medium power
(.5 to 10 mW) HeNe tubes available on the surplus market and describes the
actual laser tube which may be enclosed inside a laser head depending on the
original application:
This fabulous ASCII rendition of a typical small HeNe laser tube should make
everything perfectly clear :-).
____________________________________________
/ _________________ \
Anode |\ Helium+neon, 2-5 Torr Cathode can ^ \ |
.-.---' \.--------------------------------------. '-'---.-. Main
<---| |:::: :======================================: :::::| |===> beam
'-'-+-. /'--------------------------------------' .-.-+-'-'
Totally | |/ Glass capillary ^ _________________/ | | Partially
reflecting | \____________________________________________/ | reflecting
mirror | | mirror
| Rb + - |
+---------/\/\---------o 1.2 to 3 KVDC o-----------+
(Note: the main beam may emerge from either end of the tube depending design,
not necessarily the cathode end as shown.)
* The anode (+) end is simply a small cylindrical metal electrode with a
mirror fused or glued to its end.
The discharge at this end produces little heat or damage due to sputtering.
* The cathode (-) end is also a cylindrical metal electrode with a mirror
fused or glued to its end but in addition, there is a large 'cylindrical
can attached to the cathode and extending about half the length of the tube.
The discharge at this end is distributed over the entire area of the can
thereby spreading the heat and minimizing damage due to sputtering which
results from positive ion bombardment.
* These mirrors are not silvered or aluminized (metal coated) but are a type
called 'dichroic'. They are made by depositing many alternating layers of
hard but transparent materials having different indexes of refraction. The
thickness of each is precisely 1/2 the wavelength of the laser light (632.8
nm being the most common for a HeNe laser). This results in reflection by
interference with very high (>99.9 %) efficiency - much greater than for
even the best metal coated mirrors.
* One of the mirrors will be nearly totally reflecting and the other will only
be partially reflecting at the laser wavelength. Since the reflection peaks
at a single wavelength, these mirrors actually appear quite transparent to
other wavelengths of light. For example, for common HeNe lasers tubes, the
mirrors transmit blue light quite readily and appear blue when looking
through an unpowered (!!) tube.
* The mirrors will likely not have any 'user' adjustments. However, the
cylindrical end pieces are mounted by thinner sections of metal tubing so
that some slight changes to alignment may be possible with appropriate
fixtures. Don't be tempted: (1) grabbing the high voltage electrodes is
not likely to be pleasant and (2) it is too easy to break the seal if you
get carried away. There should be no reason for the alignment to have
changed unless you whacked the tube - it was set at the factory.
* The main beam will emerge from the partially reflecting mirror but this may
be at either end of the tube depending on model. For example, where the
tube is enclosed in a metal barrel, the HV connections will be to the anode
end and the beam will exit from the cathode end. With this arrangement, the
positive output of the power supply and ballast resistor can be very close
to the tube anode and the entire barrel can be connected to the negative
output of the power supply and earth ground.
* Since the mirrors are not perfect, there will be a weaker beam visible from
the other end if that mirror is not covered (blocked or painted over). One
use of this is to permit monitoring of laser power for purposes of optical
power regulation or other closed loop applications.
* The major discharge is forced to take place inside a thick glass capillary
tube with an inner bore of roughly 1 mm. This concentrates the discharge
forcing operation in the most common and desirable TEM00 mode.
* The cheap surplus HeNe tubes do not generally produce a fixed polarized beam.
The polarization will either be random or slowly changing as the tube heats.
Tubes with a specified polarization are also available but are generally more
costly. Lasers with external mirrors and Brewster windows will be linearly
polarized and really pricey (and more finicky).
* These tubes are nearly always operate in multimode (longitundial) with a
TEM00 beam profile. See the section: "Instant HeNe laser theory" for more
info.
* Power for a HeNe laser is provided by a special high voltage power supply
(see the section: "Basic HeNe power supply considerations" and consists
of two parts (these maximum values depend on tube size (typical 1 to 10 mW
tube is assumed):
- Operating voltage of 1,000 to 3,000 DC at 3 to 10 mA.
Like any discharge tube, the HeNe laser is a negative resistance device.
As the current *increases* through the tube, the voltage across the tube
*decreases*. The incremental magnitude of the negative resistance also
increases with descreasing current.
- Starting voltage of 5 to 12 KV at almost no current.
In the case of a HeNe tube, the initial breakdown voltage is much greater
than the sustaining voltage. The starting voltage may be provided by a
separate circuit or be part of the main supply.
See the chapter: "Helium Neon Laser Power Supplies" for more information and
complete circuit diagrams.
* With a constant voltage power supply, a series ballast resistor is essential
to limit tube current to the proper value. A ballast resistor will still be
required with a constant current or current limited supply to stabilize
operation. The ballast resistor may be included as part of a laser head but
will be external for a bare tube.
In order for the discharge to be stable, the total of the effective power
supply resistance, ballast resistance, and tube (negative) resistance must be
greater than 0 at the operating point. If this is not the case, the result
will be a relaxation oscillator - a flashing or cycling laser!
* Every HeNe tube will have a nominal current rating. In addition to excessive
heating and damage to the electrodes, current beyond this value does not
increase laser beam intensity. In fact, optical output actually decreases
(probably because too high a percentage of the helium/neon atoms/ions are in
the excited state). You can easily and safely demonstrate this behavior if
your power supply has a current adjustment or you run an unregulated supply
using a Variac. While the brightness of the discharge inside the tube will
increase with increasing current, the actual intensity of the laser beam
will max out and then eventually decrease with increasing current. (This
is also an easy way of determining optimal tube current if you have not data
on the tube - adjust the ballast resistor or power supply for maximum optical
output and set it so that the current is at the lower end of the range over
which the beam intensity is approximately constant.)
* These may be 'bare' tubes or encased in a cylindrical or rectangular laser
head - or something in between.
- Bare tubes require clip-on connections to the power supply or high voltage
connector and an external ballast resistor.
Advantages: Less expensive, discharge is fully visible resulting in an
interesting display.
Disadvantages: Fragile, exposed high voltage terminals, need to provide
your own mounting, wiring, and ballast resistor.
- Laser heads will usually come with an internal ballast resistor (though
you may still need additional resistance to match the tube to your power
supply). The high voltage cable will likely use an 'Alden' connector.
The Alden connector is designed to hold off the high voltages with a pair
of keyed recessed heavily insulated pins. This is a universal standard for
small-medium HeNe laser power supplies (the longer fatter pin is negative).
Advantages: High voltage safely insulated, wiring is already done for
you, relatively robust, easily mounted.
Disadvantages: More expensive, discharge not readily visible, repairs
to wiring (unlikely to be needed) difficult.
* The operating lifetime of a typical HeNe laser tube is greater than 15,000
hours when used within its specified ratings. Therefore, this is not a
major consideration for most hobbyist applications. However, the shelf life
of the tube depends on its construction. There are two types of (sealed)
HeNe tubes:
- Most better HeNe tubes (possibly all tubes manufactured in the last 10
years) are 'hard sealed' - the mirrors are fused to their respective
electrodes by a low temperature glass 'frit' - sort of like solder for
glass! These do not leak - at least not on any time scale that matters.
Shelf life is essentially infinite.
- Older tubes have their mirrors just glued - Epoxied to the end electrodes.
This adhesive leaks and such tubes have a shelf life of a few years - they
fail by just sitting around doing nothing. This means that a bargain tube
may not be such a bargain if it is beyond its expiration date (yes, just
like dates on milk containers) as it may have a very limited life, be hard
to start, weak or erratic, or may not work at all. You probably won't see
any of these - at least not in a working condition.
* The efficiency of the typical HeNe laser is pretty pathetic. For example,
a 2 mW HeNe tube powered by 1,400 V at 6 mA has an efficiency of less than
.025 %. More than 99.975 percent of the power is wasted in the form of heat
and incoherent light (from the discharge)! This doesn't even include the
losses of the power supply and ballast resistor.
* The most common HeNe lasers by far produce light at a wavelength of 632.8 nm
in the red-orange part of the visible spectrum - well into the region of the
human eye's high sensitivity (but not as good as green). Thus, a 1 mW HeNe
laser will appear brighter than a 4 mW laser diode operating at 670 nm.
* Green, yellow, and orange HeNe lasers are also available but are not nearly
as 'efficient' as the common red-orange type. Thus, they are also much more
costly for the same power since the spectral lines that need to be amplified
are weaker at these wavelengths and therefore, the tubes must be larger.
Typical maximum output available from 'small' sealed tubes for various
colors: green - 2.0 mW, yellow - 7.0 mW, orange - 7.0 mW, and red - 15 mW.
IR (infra-red) HeNe laser tubes are also available. However, an invisible
beam just doesn't seem as exciting!
* The width of the beam as it emerges from the tube is typically between .5 mm
and 1 mm - the inside bore diameter of the capillary discharge tube.
* The beam from a HeNe laser is already very well collimated even without
external optics (unlike a laser diode which has a raw divergence measured
in 10s of degrees). The divergence measured in milliradians (1 mR is less
than 1/17th of a degree) is usually one of the tube specifications. A small
HeNe tube may have a divergence of 1 to 2 mR. Divergence is affected mostly
by beam (exit or waist) diameter (wider is better). Other factors like the
ratio of length to bore diameter (narrower is better) may also affect this
slightly. The equation for a plane wave source is:
Wavelength * 4
Divergence angle in radians = --------------------
pi * Beam Diameter
So, for an ideal HeNe laser with a 1 mm bore at 632.8 nm, the divergence
angle will be about .806 mR. Note that although a wider bore should result
in less divergence, this also permits more not quite parallel rays to
participate in the lasing process. Also see the section: "Improving the
collimation of a laser".
TYPICAL HeNe TUBE CHARACTERISTICS
(Portions from: Steve Nosko (q10706@email.mot.com)).
The following are typical of small (bare) red-orange (632.8 nm) HeNe tubes:
Output Tube Voltage Tube Supply Voltage Tube Size
Power Operate/Start Current (75K ballast) Diam/Length
0.5 mW .9-1.0/7 KV 3.2-4.5 mA 1.1-1.3 KV 25/150 mm
1 mW .9-1.0/7 KV 4.5 mA 1.1-1.3 KV 25/150 mm
1-2 mW 1.0-1.4/8 KV 4.5-6.5 mA 1.4-1.9 KV 30/260 mm
2 mW 1.4-1.5/8 KV 6.5 mA 1.9-2.0 KV 30/260 mm
3-4 mW 1.9/10 KV 6.5 mA 2.4 KV 37/350 mm
4-5 mW 1.9/10 KV 6.5 mA 2.4 KV 37/350 mm
Melles Griot, Uniphase, Siemens, PMS, and Aerotech all show similar values.
Note that for a given optical power level, there can be substantial variation
in the tube size. Typically, longer tubes will require higher start and
operating voltages. And no, you cannot get a 3 mW tube to output 30 mW - even
instantaneously - by driving it 10 times as hard!
HeNe tubes of other colors exist but are probably rare on the surplus market.
They are not that common to begin with and are more expensive when new since
for a given power level, the tubes must be larger and thus have higher voltage
and current ratings due to their lower efficiency (the spectral lines being
amplified are much weaker than the one at 632.8 nm).
There are infrared HeNe tubes as well. Yes, you can have a HeNe and it will
light up inside (typical neon orange glow), but if there is no output beam
(at least you cannot see one), you could have been sold an infrared HeNe tube.
The IR may be visible with a video camera (assuming it doesn't have an IR
blocking filter) or bu using one of the IR detector circuits or an IR detector
card as discussed with respect to IR laser diodes.
As a side note... It is strange to see the orange glow in a green HeNe laser
tube but have a green beam emerging. A diffraction grating or prism really
shows all the lines that are in the glow discharge. Red through orange, yellow
and green, even several blue lines!! The IR lines are present as well - you just
cannot see them.
HOW CAN I TELL IF MY TUBE IS GOOD?
A variety of problems can prevent a HeNe tube from lasing properly or make it
hard to start.
* Physical damage. Obviously if the tube arrived in pieces, this is a
shipping, not a technical problem :-).
* Misaligned mirrors. Using the tube as a hammer might bend the mirror mount
at one end or the other. There are ways of determining and adjusting this
alignment but they require some optical components and special jigs. Without
these, adjustments are hit or miss (mostly miss) and will more likely result
in a broken seal than anything else :-(. Problems with mirror alignemnt are
very unlikely to occur with these tubes unless you were working hard at it!
* Loss of gas fill. This may result in a total inability to start or sustain
the discharge. There is usually a metal sealing nipple at one end. This
might be damaged.
* Incorrect gas fill. There may be a glow but the laser output will likely
be weak or non-existent. The normal color of the discharge is whitish
red-orange - a somewhat unsaturated version of the red-orange glow of a
(true) neon sign.
- Loss of helium (from diffusion through the glass or seals) will result in
the glow becoming deeper red-orange and less white. There will be little
or no emission at the wavelength of helium's spectral lines. It probably
isn't worth the effort to refill but see the section: "Recharging HeNe
tubes".
- A leak which has allowed some air to enter (but where it is not totally
up to atmostpheric pressure) will result in a glow with a white or pink
color. Depending on the actual pressure, the intensity will vary as well.
If you can sustain a discharge but it is the wrong color (weak or white/pink
color), you may have one of those really old Epoxy sealed tubes that leak
and air has leaked in. Again, probably not worth repairing.
* Damaged electrodes or mirrors due to running with the power supply polarity
reversed or greatly excessive current for a prolonged period of time. I
don't know exactly what the physical effects might be but I would suspect
metal sputtering from the negative electrode coating the mirror at that end
of the tube. Buy another tube.
IMPROVING THE COLLIMATION OF A HeNe LASER
The following applies to any laser which outputs a substantially parallel
beam but is written specifically for HeNe lasers. Collimation of laser
diodes require a slightly different approach - see the section: "Divergence
of laser diodes".
Although the divergence of a HeNe laser is already pretty good without any
additional optics, the rather narrow beam as it exits from the tube (typically
1 mm) results in a beam with a typical divergence between 1 to 2.5 mR (order
of 1 mm per meter or worse) if no other optics are used.
As noted in the section: "HeNe laser operation", beam divergence is inversely
proportional to the beam diameter. Thus, it can be reduced even further by
passing the beam through a pair of positive lenses - one to focus the beam to
a point and the second to collimate the expanded beam.
A small telescope can be used to collimate a laser beam and will be easier to
deal with than individual lenses. (This is how laser beams are bounced off
the moon but the telescopes aren't so small.) Using a telescope is by far
the easiest approach in terms of mounting - you only need to worry about
position and alignment of two components - the laser tube and telescope.
If you want to use discrete optics:
* The focusing lens should have a short focal length (F1) such as a microscope
or telescope eyepiece (e.g., F1 of 10 mm) or low power microscope objective
(e.g., 10X). Note: the objective lens from a dead CD player has an ideal
focal length - about 4 mm - but is aspheric and would probably not be that
great but give it a try!
This will focus the laser beam to a (diffraction limited) point F1 in front
of the lens from which it will then diverge.
* The collimating lens should be a large diameter medium focal length (e.g.,
15 mm D2, 100 mm F2) lens placed F2 from the focus of the first lens.
For optimal results, the ratio of collimating lens diameter to focal length
(D2/F2) should greater than or equal the ratio of HeNe beam diameter to
focusing lens focal length (D1/F1). This will ensure that all the light
is captured by the collimating lens.
The beam will be wider initially but will retain its diameter over much longer
distances. For the example, above, the exit beam diameter will be about 10 mm
resulting in nearly a 10 fold reduction in divergence.
Adjust the lens spacing to obtain best collimation. A resulting divergence of
less than 1 mm per 10 meters or more should be possible with decent quality
lenses - not old Coke bottle bottoms or plastic eyeglasses that have been used
for skate boards :-).
HeNe LASER TUBE LIFE
Neon signs last a long times - years - how about HeNe laser tubes?
Lifespans of HeNe laser are long - 20,000 or more operating hours are typical.
Shelf life of non-hard sealed tubes is limited by diffusion of the Helium
atoms. Helium atoms are slippery little devils. They diffuse through almost
anything. In the case of HeNe tubes, diffusion takes place mostly through the
Epoxy adhesive used to mount the mirrors in non-hard sealed tubes (not common
anymore) and through the glass itself but at a much much slower rate.
The gas doesn't 'wear out'. However, excessive or reverse current can degrade
performance after a while. Electrode material may sputter onto the adjacent
mirrors or excessive heat dissipation may damage the electrodes themselves.
A HeNe tube, when properly connected has much of its heat dissipated by the
bombardment of positive ions at the cathode (the big can electrode) which is
made large to spread the effect and keep the temperature down. Hook a tube up
backwards and you may damage it in short order!
RECHARGING HeNe TUBES
"I have two large tube green HeNes and and a 1 micron IR HeNe that are dead
from obvious low helium pressure (spectrum from grating shows only weak He
lines) has anyone had any success with putting tubes in a pressure chamber
filled with Helium so it diffuses the other way?"
HeNe tubes which do not laser well or at all due to loss of helium can
sometimes be rejuvenated by soaking them in helium at normal atmospheric
pressure for a few days or weeks.
The point to realize is that it is the partial pressure of each gas that
matters. Neon is a relatively large atom and does not diffuse through the
tube at any rate that matters. However, helium is able to diffuse even
when the pressure difference is small. Even for a HeNe tube at 2 Torr, the
partial pressure of helium is much greater than its partial pressure in the
normal atmosphere. So, helium leaks out even though the total pressure
outside is several hundred times greater. Conversely, soaking a HeNe tube
in helium at 1 atmosphere will allow helium to diffuse into the tube at several
hundred times the rate at which it had been leaking out. Thus, only a few
days of this treatment may be needed if the problem is low helium pressure.
This hardly seems worthwhile for a $25 1 mW HeNe tube but it is something to
keep in mind for other more substantial types.
(From: Mark W. Lund (lundm@xray.byu.edu)).
I have rejuvenated HeNes with low Helium pressure. Since the partial pressure
of 1 atmosphere helium is much higher than inside the tube you don't really
need to use high pressure, or even increased temperature. I just put them in
a garbage bag and blasted some helium into it from time to time. The length
of time necessary in my case was a few days, but depending on the glass type,
thickness, and sealing method this may vary. It would be good to test the
power every couple of days so you don't overshoot too much.
One warning, helium has a lower dielectric strength than air, so don't try to
operate the laser in helium, it may arc over.
HELIUM NEON LASER POWER SUPPLIES
BASIC HeNe POWER SUPPLY CONSIDERATIONS
You will need 1,100 to 2,400 VDC at a few mA and an 5 to 12 KV starting voltage
(but almost no current). Precise values depend on the size of the tube. This
assuming something in the .5 to 10 mW range - not a 250 mW monster. Unlike
laser diodes, the HeNe drive is not nearly as critical to performance and tube
life :-). Therefore, even your tube did not come with a datasheet, you can
probably guess fairly closely as to its requirements.
There are any number of ways of constructing these supplies - over a dozen
sample circuits are provided in the chapter: "Complete Helium Neon Laser Power
Supply Schematics". However, you don't need to buidl your own:
* If you just want a working laser, buying a power supply may be worth the
money - these can be had for about $25 (typical for 1 to 2 mW tubes) to
$100 depending on size. These are available both new and surplus.
With these, at most you will have to add a ballast resistor and power line or
battery connections. Models are available that run off either low voltage
DC (regulated is desirable) or 110 or 220 VAC.
See the section: "Examples of the use of commercial power supply bricks".
* Kits are also available but may not be any cheaper and are not necessarily
as well designed as surplus commercial units.
* The complete laser and optics assemblies from supermarket checkout UPC
scanners and other similar devices are often available at very reasonable
prices - $50 to $100 - which includes the HeNe laser tube, power supply,
various lenses and mirrors, scanner motor or galvo, and other neato stuff.
* HeNe tubes and power supplies are quite frequently offered by people posting
on the sci.electronics hierarchy, sci.optics, alt.lasers, or other USENET
newsgroups. Prices are often very low but of course you may have no way
of knowing who you are dealing with.
If you still want to build your own, there are basically two approaches for
the operating voltage (AC line operated or high frequency inverter) and three
approaches for the starting voltage (diode/capacitor multiplier, pulse trigger
circuit, or high compliance design).
Here are six options for providing the operating voltage. These examples
assume an output of at least 1,800 VDC:
1. Use a 1,300 VRMS power transformer. Then, all that is needed is a rectifier
and filter capacitor.
2. Use a 700 VRMS power transformer with a 2 diode 2 capacitor voltage doubler.
3. Use a lower voltage power transformer and a multi-stage voltage multiplier.
Up to 6 stages should be reasonably easy to construct.
4. Build a low voltage input inverter using a flyback transformer from a small
B/W or color TV computer monitor, or video terminal but running at lower
voltage than normal. These usually have a built in HV rectifier but you
will need a HV filter capacitor and ballast resistor. While rated at only
a mA or so for the CRT HV, more current should be available at reduced
voltage. With proper design, it is possible for there to be enough voltage
compliance to be self starting. See the section: "Sam's inverter driven
HeNe power supplies".
5. Build a HV inverter based on any of a number of simple DC-DC converter
topologies. See the document: "Various Schematics and Diagrams" for ideas.
You will probably need to modify these mostly by increasing the number of
turns on the output windings of the inverter transformer - which in itself
may be a challenge to maintain low capacitance and high voltage insulation.
See the section: "Simple inverter type power supply for HeNe laser" for one
modified for driving a HeNe laser tube.
6. Build a HV inverter of the type discussed above using a PWM controller
integrated circuit - Linear Technology, Maxim, Motorola, National
Semiconductor, Unitrode, and others have suitable DC-DC controller chips.
I would recommend (1) or (2) if portability is not a big issue and you can
locate a suitable transformer. These are virtually foolproof (well, at least
as long as you don't fry yourself from the high voltage). For small tubes,
the design described in the section: "Edmund Scientific HeNe power supply"
is about as simple as possible.
See the sections: "AC line operated power supplies" and "Inverter type power
supplies" for more discussion of features, basic operation, and design issues.
A typical configuration is shown below. As noted, the starting circuit may
be omitted in a high compliance design. The regulator is desirable but the
location shown (low-side series) is just one option. Without a regulator,
tube current will need to be set by controlling the power input and/or
selecting the ballast resistor.
+----------+ +------------------+ HV+ Ballast Resistor
o------| |-----| Starting Circuit |--------------/\/\----+
Input, | Main | +------------------+ Rb |
AC line | Power | |
or DC | Supply | +-----------+ Tube- +-----------+ Tube+ |
o------| |-----| Regulator |----+----|-| -|-------+
+----------+ HV- +-----------+ _|_ +-----------+
- HeNe tube
Depending on the open circuit voltage of your power supply, a ballast resistor
in the 30K to 150K ohm range will be essential to limit the current to the
value specified for your particular tube. A higher voltage supply and larger
ballast resistor will be more stable if there is no built-in regulation. This
results from the fact that the voltage drop across the tube is relatively
independent of tube current so it subtracts out from the supply voltage. What
is left is across the ballast resistor and changes by a proportionally greater
amount when the line voltage varies. The smaller the ballast resistor, the
more this will affect tube current.
You really do want the current to be close to that recommended for your tube.
This can be accomplished by adjusting either the input voltage to the power
supply or the output current and/or by selecting the value of the ballast
resistor. Excessive current is bad for the tube and will actually result in
decreased optical output. It is not possible to pulse a HeNe laser for higher
power. Without any regulation, the value of the ballast resistor is more
critical and power line fluctuations will significantly affect tube current
though such variations may not matter for non-critical applications. Note that
since the tube itself provides a relatively constant voltage drop around its
nominal current, a small change in line voltage will affect the tube current to
a much greater degree than would be expected by the percent of the actual
voltage change. The incremental change in tube current will be closer to:
delta(I) = delta(V)/Rb where Rb is the ballast resistor. Therefore, a
regulator is often desirable.
As noted, the large (can) electrode is the cathode (-) end. The tube will
probably run if connected backwards but the small anode (supposed to be
positive but is incorrectly connected as negative) will get very hot since
most of the heat dissipation is at the negative electrode due to positive ion
bombardment. Tube life will likely be shortened.
A variety of sample circuits are provided in the chapter: "Complete Helium
Neon Laser Power Supply Schematics".
For detailed plans, see the books listed in the section: "References on laser
principles, technology, construction, and applications".
IDENTIFYING THE RANGE OF A POWER SUPPLY AND SELECTING THE BALLAST RESISTOR
This may be necessary to select the ballast resistor to match a HeNe tube up
with a power supply that is not marked or even one that is - the ratings on a
typical HeNe power supply do not tell you how it will behave under varying
load conditions. Some are current controlled while others are not.
It is difficult to measure the output voltage of a HeNe laser power supply with
a multimeter even if it is supposedly within your meter's range. Connecting
the meter across the tube while it is on will likely extinguish the arc due to
the capacitance of the probe inducing a momentary dip in the voltage. Leaving
a VOM or DMM connected during starting may prevent the tube from firing due to
the multimeter's additional capacitance and reduced resistance. And, it may be
damaged due to arcing from any high voltage starting pulses. A VOM or DMM
with a suitable high voltage probe can be left connected to a wide compliance
type power supply and possibly on one using a voltage multiplier (though it
may load it excessively) but should not be used with a trigger pulse type
starting circuit.
Selecting a ballast resistor that works with a given tube is usually a trivial
exercise so don't let the length of the following discussion intimidate you!
In fact, if your power supply is variable or current regulated, a 75K resistor
will almost always work just fine. It turns out that the value of the negative
resistance of a typical HeNe tube around its optimal operating point is usually
in approximately 50K. The reason is that larger HeNe tubes tend to have longer
but wider bores and the effects of these tend to cancel out. Using a 75K
ballast resistor provides adequate margin without dissipating more power than
needed.
* One way to select the ballast resistor is by using an adjustable dummy load
in place of the tube. This approach can also be used to determine the range
of your power supply so that you will know what tubes it can drive reliably
without risking your HeNe tubes at all.
Use a voltage divider constructed from a 500K potentiometer (actually four
100K (5 W) resistors and five 20K, (1 W) resistors to get high enough
wattage - not an actual pot), 150K (5 W) resistor and 1K 1/2 W (sense)
resistor.
This load will fool any auto start circuit into thinking the tube is running.
These resistor values should work for most tubes rated between 1100 V and
2,400, and currents between 4 mA and 8 mA with typical power supplies but
yours may be the exception. Therefore, the resistor values may need to be
adjusted if you cannot obtain meaningful results.
Connect the components as follows:
+----+ +--o Measure o--+
| | | |
R1 v | R2 v R3 v
PS + o-------/\/\--+-----/\/\------+------/\/\-----+------o PS -
500K 25 W 150K 5 W 1K
Measure the voltage across R2 to determine current. The sensitivity will be
1 V/mA. Alternatively, simply put a 10 or 20 mA current meter across or in
place of R3. (Then, R3 is effectively 0 for the calculations, below.)
- Start with R1 at maximum.
- With the power supply running, adjust R1 until the voltage across R3 is
equal to 1,000 x Io (1 V/mA) where Io is the desired operating current.
Note: Although shown as a pot, R1 will need to be constructed from several
smaller power resistors. For higher output power supplies, you will need
to create this from a string of four 100K, 5 W resistors and five 20K, 1 W
resistors. Start high and reduce the resistance until you are able to
obtain the desired output voltage across R3. Caution: Shut down, pull the
plug, and confirm that the power supply has discharged before touching any
resistors!
- Shut down, confirm that the output of the power supply has discharged, and
without turning the knob, measure the resistance of the entire R1 combo.
- The value of the ballast resistor should then be:
Ballast resistor Rb = R1 + 150,000 + R3 - (Vo/Io);
where Vo is the operating voltage from the tube specifications.
* Start with a very high value ballast resistor in series with the power
supply, tube, and a 1K ohm sense resistor. Gradually reduce the value of
this temporary ballast resistor (substituting smaller resistors, removing
resistors, and/or adjusting a pot) until you measure the correct current
from Io = V/1,000 (where V is measured across the sense resistor or
directly). Then replace the temporary ballast resistor with one or more
fixed resistor.
You can leave the sense resistor in the *return* of the power supply (in the
cathode circuit) to monitor current and confirm that your ballast resistor
is satisfactory. See the chapter: "Complete Helium Neon Laser Power Supply
Schematics" for more information and sample circuits.
* For a line operated power supply, you can use a Variac and guess at a ballast
resistor - say 75K to start. Then modify its value until you obtain the
desired tube current at full line voltage.
* For a regulated power supply, the optimal ballast resistor will operate the
tube in the middle of the voltage compliance range of the regulator.
Don't get carried away - running the tube with slightly excessive current
won't damage or destroy it immediately (unlike a laser diode). Therefore,
selecting the ballast resistor can even be done by (reasonable) trial and
error or by simply maximizing output beam brightness!
Some laser power supplies even come with a built in sense resistor in a readily
accessible relatively safe location for monitoring tube current. Or, you can
always add such a feature. See the section: "Enhancements to Aerotech model
PS1 HeNe Power Supply".
Here are a couple of approaches to selecting the ballast resistor for a small
tube and power supply like those described in the section: "Edmund Scientific
HeNe power supply".
(From: Steve Nosko (q10706@email.mot.com)).
The ballast resistor, the actual voltage at the doubler output and the tube
running voltage determine the tube current. Look at the tube ratings for the
running voltage and tube current - 1,150 V at 4 mA for the tube I was using.
One can approach the design from two directions:
* With a supply of 1,750 V and a tube voltage of 1,150 V. this leaves 600 V.
across the ballast resistor. Once you get the transformer picked out, make
your best estimate of the output voltage you'll have. It should be above
1,400 V for a 0.5 mW tube. Any less and you could get into trouble since the
ballast resistor should be kept above 50K. Some sources recommend 70K as an
optimal value for a .5 mW tube.
* Working the other way. First calculate the ballast resistor voltage drop
as, V = tube current * 70K. Add this voltage to the tube voltage to get the
required supply voltage. This would be a minimum supply voltage for that
tube. If yours gives a few hundred volts more, you can make the ballast
resistor larger to obtain the required tube current. It's just that the
bigger the ballast resistor, the more power you throw away. If you think
you may go to a more powerful tube in the future, then get a higher voltage
transformer output for the bigger tube and just use the larger ballast
resistor for now.
STARTING PROBLEMS AND HARD-TO-START TUBES
Some tubes seem to practically start on their own. Other won't perform even
when you stand on your head, hold your breath, and provide the proper chants
and sacrifices :-). Or, your power supply operating voltage, ballast resistor,
and other factors may need modification.
There are two types of problems. You need to determine if the discharge is
being initiated at all. If the starting voltage is adequate, there will be
momentary flashes that may be extremely short and weak and only visible in a
darkened room. As you approach a stable condition, these will become brighter
and longer. At the hairy edge, you may get a nice flashing laser!
1. Tube does not fire at all - there is no evidence of any beam, even for an
instant. While tube manufacturers generally specify a starting voltage of
7 to 10 KV (or higher), typical tubes will fire with 3 to 5 times their
operating voltage. Thus, a tube that runs on 1,700 VDC will probably start
on 5,400 to 8,500 VDC.
No action at all generally means the starting voltage is inadequate for the
tube, there are other circuit problems, or the tube is bad. Tubes with
longer and narrower bores (capillaries) will generally require greater
starting voltage.
There may be too much leakage in the anode circuit preventing the buildup
of adequate starting voltage. For pulse type starters, there may be too
much capacitance as well.
If you need to increase the input to start or obtain any sort of response
but then must back it off substantially to reduce the tube current to the
proper value, low starting voltage or one of the other related problems is
indicated.
Assuming the power supply and wiring check out and the tube is good, the
only solution is to boost the starting voltage or use a different type of
starting circuit (pulse trigger versus voltage multiplier or vice-versa).
2. Tube flashes momentarily but does not 'catch'. What happens is that the
discharge is initiated but the voltage drops too much at the tube anode
and the discharge goes out.
To produce a stable discharge, the following must be satisfied:
* The sum of the effective resistance of the power supply and the ballast
resistor and the (incremental) negative resistance of the tube (dV/dI at
the operating point) must be greater than 0.
* The voltage across the tube must be above the minimum for the tube at the
operating current.
* The current must be above the minimum for the tube/power supply/ballast
resistor combination.
These factors are not independent. Since the negative resistance and
sustaining voltage of the tube are not normally specified and depend on
current, some amount of trial and error may be required to achieve
consistent stable operation but in most cases it really is very easy.
Cycling behavior can be due to several factors:
* Poor power supply voltage regulation or excessive ripple. Until the
tube fires, there is essentially no load on the supply resulting in much
greater voltage than under load. Except for a high compliance type of
design where this is needed to produce the starting voltage, minimizing
this difference will improve stability and reduce the voltage needed for
stable operation.
If the transformer or inverter drops too much under load, the tube voltage
may fall below the minimum for the tube/ballast combination as soon as it
starts. This cycle will repeat continuously or it occasionally may catch.
Use a higher voltage and larger ballast resistor, and/or increase the uF
value of the main filter capacitor (and/or the one in the DC supply to an
inverter type supply as well if it isn't regulated).
Minimum capacitor values for less than 5 percent voltage ripple:
- Line operated supplies: .5 to 1 uF.
- Inverter output: .005 to .01 uF (10 KHz).
- Unregulated inverter input: 15,000 to 20,000 uF (12 V, 1 A).
Actual ripple in the current to the tube may be several times greater
than this since it depends on the change in voltage with respect to the
total effective resistance of the PS+tube+ballast resistor combination).
However, the resulting ripple in the optical output power will be 2 to 10
times lower than the ripple in the current depending on operating point.
The lowest will occur around the tube's optimal current specification.
* Ballast resistor too large for the operating voltage. The operating
current falls too low resulting in increased (magnitude of) negative
resistance. Once the total system resistance goes negative, the
discharge becomes unstable and goes out. The result is a flashing laser
like a neon bulb relaxation oscillator.
For an unregulated power supply, increase the operating voltage and/or
decrease the ballast resistance.
For a regulated power supply, decrease the ballast resistance so that the
voltage for the desired operating current falls within its compliance
range.
* Too much stray capacitance/wire length in anode circuit. The system is
behaving like a relaxation oscillator as the capacitance charges and then
discharges through the tube.
Shorten the wiring - minimize the distance - between the ballast resistor
and tube anode and don't use high voltage coax.
EXAMPLES OF USE OF COMMERCIAL POWER SUPPLY BRICKS
The simplest approach to powering HeNe tubes is often to purchase surplus
or new power supply 'bricks' - fully self contained inverter type (usually)
power supplies from one of the suppliers listed in the section: "Parts
Sources" or elsewhere. These are compact, high efficiency, and reliable.
Cost may be anywhere from $25 to $100 or more depending on power capability,
whether new or surplus, and other factors. They may also be contained along
with a HeNe tube, ballast resistor, and wiring as a complete laser optics
assembly. These have become available at very attractive prices as products
like UPC scanners and laser disc players have switched to laser diodes. Since
nearly everything but a wall plug is likely included in such a package, this
approach will result in a working laser with minimal effort.
The following sections describe the required connections and additional
circuitry that I used to make complete lasers using two types of HeNe tubes
and power supplies that were available from Herbach and Rademan.
1 mW HeNe LASER POWERED BY 12 VDC, 1 A WALL ADAPTER
H&R part numbers: power supply - TM91LSR1495, HeNe tube - TM94LSR2631.
This uses a short (150 mm) HeNe tube and power supply running off of 9 VDC.
I built these into a case which was from a 1/8" cartridge tape backup system
in its former life. In order to obtain regulated 9 VDC, an LM317 IC regulator
on a heat sink was added along with a power switch, power-on LED, and the
required ballast resistor of 150K. The ballast resistor was determined by
monitoring the HeNe tube current and selecting values until the current was
correct. The HeNe tube was mounted on standoffs using a pair of nylon cable
clamps and aimed through a hole drilled in the plastic case.
S1 +-------+ In+ +--------+ HV+
Vin+ o--/ --+---| LM317 |---+-------+-------+-------| |-------+
Power | +-------+ | | | | | |
| | A / | / R3 | | / Rb
| | \ R1 | \ 500 | 9 VDC | \ 150K
| | / 240 | / | HeNe | /
_|_ C1 | | +_|_ C2 | | Laser | | Tube+
--- .1 +-------+ --- 10 | | Power | .-|-.
| uF | - | uF __|__ IL1 | Supply | | |
| \ R2 | _\_/_ LED | Brick | | | LT1
| / 1500 | | | | | |
| \ | | | | ||_||
| | | | Gnd | | HV- '-|-'
Vin- o------+-------+---------------+-------+-------| |-------+ Tube-
+--------+
2 mW HeNe LASER USING AC LINE OPERATED POWER SUPPLY BRICK
H&R part numbers: power supply - TM92LSR2278, HeNe tube - no longer listed,
would have been similar to the 2 mW tube listed in the section: "Typical HeNe
tube characteristics" with ballast resistor and Alden connector.
This was a power supply and HeNe tube combination. This brick can be wired
for either 115 VAC or 230 VAC operation and includes an 'enable' input which
must be pulled to around +5 V to turn on the tube. Rather than using a
separate power supply just for this, I provided a battery holder with 4 AA
cells. Even old tired decrepit ones work fine in this application! The only
other parts I added were the line cord, power switch, fuse, light, and enable
switch.
The ballast resistor was already built into the tube mounting so that this
was truly a 'plug-and-play' assembly.
S1 _ F1 AC1 +----------+ HV+
H o---/ ---- _---+--------------------+-----| |--------<<---+
Power .5 A | | AC3 | | red |
/ R1 +-----| | / Rb
\ 47K S2 Enable Ebl | 115/230 | \ 75K
/ +-----/ ---------| VAC HeNe | /
| | | Laser | | Tube+
+|+ | | | | | Gnd | Power | .-|-.
|o| IL1 +---||||||||-----| Supply | | |
|o| NE2H +| | | | - | Brick | | | LT1
+|+ Power On B1 5-6 V | | | |
| +-----| | ||_||
| | AC2 | | black '-|-'
N o--------------+--------------------+-----| |-----+--<<---+ Tube-
AC4 +----------+ HV- |
|
G o----------------------------------------------------------+
AC LINE OPERATED POWER SUPPLIES
These are quite simple consisting of a high voltage transformer, rectifier
or doubler, filter capacitor stack, starting circuit, and optional current
regulator. For details, see the chapter: "Complete Helium Neon Laser Power
Supply Schematics".
The transformer output generally feeds a half wave rectifier or 2 diode 2
capacitor doubler and filter capacitor stack.
Either a parasitic voltage multiplier or pulse trigger type starting circuit
can be used with these designs.
Compared to inverter type power supplies, line operated units are easier to
construct (no custom transformer is needed) and troubleshoot. Of course, they
are not nearly as portable in two ways: the power transformers you are likely
to find are usually quite heavy and there is that annoying line cord to drag
around!
However, most of the components are readily available or can be constructed
from common parts including the high voltage diodes and capacitors. The only
problem may be the power transformer which is typically 600 to 1,200 VRMS at
20 mA or so:
* The power transformer from an old tube type TV or audio amplifier would be
suitable (though gross overkill - probably 5 to 10 times the current that is
actually needed for a typical power supply). Check your attic :-).
* The output voltage can usually be adjusted up to +/-10% of its rated value
by using the (otherwise unneeded) filament windings (5 and 6.3 VRMS typical)
in series with the *primary* in phase (decreases output) or out of phase
(increases output). See the section: "Boosting the output of a transformer
with multiple secondary windings".
* You may be able to stretch this another 20 percent or so by connecting the
secondary of a separate low voltage power transformer in series with primary
of the HV transformer in or out of phase as above.
Caution: Do not be tempted to increase the high voltage output of a power
transformer by more than 30 percent or so above it rated value (by either
driving its primary with a higher than rated voltage or by adding booster
windings in series with the primary). Even this may be excessive depending
on its design margins. At some point core saturation will result in a
dramatic increase in input current, overheating, meltdown, smoke, 6 foot
flames, etc.
In addition, the insulation ratings may be inadequate for the increased high
voltages now produced by the secondary.
Thus using a 110 V transformer on 220 V to obtain double the output is
probably not a good idea though I know people who have done this and lived
to tell!
* The high voltage transformer can also be constructed from more than one
transformer. You can put two or three secondaries in series, but don't get
carried away since a lower voltage transformer may not be insulated for the
high voltages (several KV) present in a HeNe power supply.
For example, using two 380 VRMS transformers in series will result in over
2,000 VDC without playing games and 2,200 to 2,500 V with one of the booster
techniques described above.
* I DO NOT recommend the following: oil burner ignition transformers, neon
sign (luminous tube) transformers, microwave oven transformers, utility pole
transformers, and 100 KVA substation transformers :-). These would likely
result in designs that were impractical, excessively expensive, or extremely
dangerous.
- Oil burner ignition transformers provide about 10 KV at 15 or 20 mA.
This voltage is way too high at normal line voltage and at reduced line
voltage, the current is inadequate.
- Neon sign or luminous tube transformers (same thing) provide 12 to 15 KV
at 15 to 60 mA (some much higher). This is way too much voltage and way
too much current. They may be usable at reduced line voltage but are
really too large to be practical. Yes, even though you may find the words
'neon' and 'tube' in their names, these beasts should be avoided (for this
purpose at least).
- Microwave oven transformers produce the required output voltage (1,500
to 2,500 V) but are capable of very high current at high voltage - over
an Amp. This is an instantly lethal combination which means they are
just too dangerous to consider. Don't even be tempted!
- Utility pole and substation transformers. Aside from requiring a fork
lift or 10 ton crane to move, I don't think the power company would be
happy if one of these were to disappear one night :-).
If you have the option of obtaining a slightly higher voltage transformer than
you actually need, go for it! Then, if you acquire a higher power tube in the
future, you will be all set. For now, it will just require a larger ballast
resistor or Variac to run at reduced input voltage.
It might be worth trying a TV or audio equipment repair shop - they may have
spare transformers from old tube sets laying around gathering dust. These
are ideal and can probably be had for next to nothing.
Another option is an electronics surplus supplier - I have seen suitable
transformers at some of these in the past but don't know what is currently
available.
A 3 or 4 stage voltage multiplier could be used to boost the output of a lower
voltage transformer if a suitable high voltage transformer cannot be located.
However, to obtain the needed current, the capacitors would need to be quite
large - perhaps 1 uF at 1,000 V or more. Also, you would then probably need
to use a pulse trigger type starting circuit as a multiplier type starting
circuit may not be able to provide enough output with a reasonable number of
stages since the available p-p input voltage will be less with this approach.
I have recently been using the power transformer from a long dead tube type TV
both for testing a higher power commercial HeNe supply board and as the basis
for a power supply of my own. See the section: "Sam's line powered HeNe laser
power supply". There was a selector for line voltage adjustment built into
the transformer. With this set for lowest line voltage (and thus highest
output) and the filament windings connected out of phase, it produces over 900
VRMS at 115 VAC input and over 1,150 VRMS using a Variac that goes up to 140
VAC. This translates into a doubled DC voltage of 2,500 to 3,000 VDC - more
than ample for most HeNe tubes up to 10 mW.
AC INPUT CIRCUITRY FOR HeNe POWER SUPPLIES
For safety and protection, you will need a fuse, line switch, and power-on
indicator at a minimum:
_ F1 S1 T1 or T100
Hot o----------- _----------/ -------+------+ +----o X
1 A Power | | ||(
R0 / + ||(
47K \ )||(
/ )||(
| )||( HV Secondary
IL1 +|+ )||(
NE2H |o| )||(
Power On |o| )||(
+|+ + ||(
| | ||(
Neutral o-------------------------------+------+ | +----o T
|
Ground o----------------------------------------+-------o Tube- (HV Return)
_|_
-
Important: Use a grounded (3 wire) line cord and connect earth ground to the
case if it is made of metal, transformer core, and high voltage return of the
tube (Tube- on the schematics below). This will assure that the tube housing
is grounded and that no fault will result in the user accessible parts becoming
electrically live as long as the line cord is plugged into a properly grounded
outlet. The alternative is to double insulate everything but this may be
impossible if you are using a commercial laser head where the tube cathode
is already connected to its metal shell.
BOOSTING THE OUTPUT OF A TRANSFORMER WITH MULTIPLE SECONDARY WINDINGS
If you are using a transformer from a tube type TV or amplifier and want to
boost the output a bit, determine how to connect the unused filament windings
as follows. (This assumes you have a typical pair of filament windings on
your transformer):
1. Temporarily connect the 5 V (yellow) and 6.3 V (green) windings in series.
Power the transformer and measure the output across the ends of this
'booster' winding.
* If it measures around 11.3 V, proceed to (2).
* If it measures around 1.3 V, interchange the yellow or green wires but
not both.
2. Power the transformer and measure the high voltage winding. Caution: This
is probably over 700 V with significant current available - take care. Make
a note of your reading and then disconnect power.
3. Temporarily connect the booster wiinding from (1) in series with the
primary. Power the transformer and measure the high voltage winding.
* If the high voltage now reads higher than in (2), you are finished.
* If it measures less, disconnect power and reverse the connections to the
to the booster winding.
4. Use Wire Nuts(tm) or solder to premanently connect the windings in the
desired configuration.
This approach should result in an increase by a factor of 115/104.3 or about
10 percent. Use a similar procedure to decrease the voltage. Depending on
your needs and your particular transformer, adjust appropriately.
Alternatively, you can probably safely achieve up to a 25 or 30 percent boost
using a separate low voltage power transformer to provide your booster winding.
(Start with step (2).
For example, with a 24 V transformer, a 26 percent increase in output voltage
will result - this is probably about the limit before you risk core saturation
with a typical transformer but your mileage may vary.
Caution: On transformers with dual primary windings (to support 115 or 230
V power), it is possibly in principle to use one of these to drive the supply
and the other as a booster on the secondary side. I Do not recommend this
approach as the insulation between the two primary windings may be inadequate.
INVERTER TYPE POWER SUPPLIES
Most modern designs are of this type since they can be made small, light
weight, and efficient. However, component selection and procurement can
be an issue for hobbyists and producing a reliable system can be more of
a challenge than for AC line operated supplies.
Several types are possible:
1. Power oscillator inverter using a variety of simple configurations. These
usually require either a custom wound ferrite transformer or a modified
flyback transformer. Aside from the transformer, these are extremely simple
requiring typically a half dozen components - all readily available.
However, self oscillating designs are generally not as efficient as driven
ones (see below) and may be unstable under certain load conditions.
2. Driven inverter using a separate oscillator (e.g., 555 timer) to switch
current to a stepup transformer. As with (1) above, the transformer can
be custom or a modified flyback. These inverters can be made very flexible
with adjustable frequency and duty cycle as well a feedback loop for
regulation.
3. Inverter using controller chips and/or power devices from companies like
Linear Technology, Maxim, Motorola, and others. For these, application
notes will be available but modifications will usually be needed to adapt
their circuits for the required high output voltage.
With any of these, the starting circuit can be separate (a voltage multiplier
or pulse trigger type) or built in as part of a high compliance design.
For DIY projects, it is best to run the inverters from low voltage DC. While
it is also possible to build inverters that operate directly from the power
line (commercial power supplies often do this) with just rectification and
filtering, I DO NOT recommend this as an option here for two reasons: (1)
there is a significant added level of danger when dealing with the testing of
line connected circuitry and (2) it is much easier to blow up mountains of
power transistors and other components when running at 150 VDC compared to
12 VDC before a design is perfected!
See the chapter: "Complete Helium Neon Laser Power Supply Schematics" for
sample circuits of types (1) and (2).
For an example of (3), HeNe laser drive circuitry is briefly covered in a
Linear Technology Corp. application note: AN-49, p.13. This is a low voltage
DC powered circuit using an LT1170 chip for fully automatic starting and
feedback control of operating current. It is essentially a constant current
supply with a voltage compliance range of 10 KV. HeNe tube power requirements
are also discussed. Unfortunately, the special transformer may not be readily
obtained.
STARTING CIRCUITS FOR HeNe LASER TUBES
The objective is to achieve the required breakdown voltage of the laser tube
and then disappear into the woodwork. Almost no energy is required - just
voltage. Therefore, any method that can achieve this will be adequate.
Conceivably, even walking across a wool carpet on a dry day and touching the
anode of the tube would make a suitable starting procedure (perhaps assuming
you don't die from the high voltage - this depends on *your* requirements).
However, higher tech approaches are usually preferred :-). In fact, the piezo
assembly from a disposable lighter or gas grill can be used as the basis for a
starting circuit. See the section: "Piezo impulse starting circuits".
Although usually specified by tube manufacturers to be 7 KV to 12 KV depending
on size, the actual starting voltage is often much less, typically only 3 to 5
times the operating voltage. However, to be sure, adhering to the minimum
starting voltage specifications is still a good idea unless you intend to only
use one tube and test it before constructing your power supply.
Here are four possibilities For the starting voltage:
1. Use a 'parasitic' voltage multiplier. This is an n-stage diode-capacitor
ladder operating off of the (unfiltered) output of the primary supply. This
can be used on both high frequency inverters and line powered systems though
component values will differ. Since starting the tube requires essentially
no current, the capacitors are small in either case. The voltage multiplier
goes essentially in series with the operating supply and is effectively
shorted out once the tube fires.
A voltage multiplier can be constructed from common diodes and capacitors
relatively easily. Alternatively, a 3X or 4X type may be purchased as a
replacement for the type found in some color TVs and monitors - you may
even have one gathering moss in your junk box!
See the section: "Voltage multiplier starting circuits".
2. Use a trigger transformer such as found in a photographic strobe to pulse
the tube. Its secondary would go in series with the positive side of the
main supply. A push button can be used to start the tube or a simple
automatic circuit which senses that there is no tube current flowing can do
it for you. However, a much larger trigger transformer than found in a
disposable pocket camera will likely be needed to start it in one shot. A
small TV or monitor flyback transformer or automotive ignition coil can
also be used. See the section: "Starting circuits using pulse or flyback
transformers".
3. Use the piezo assembly from a disposable lighter or gas grill as a source
of a high voltage pulse. The only additional parts required are a pair
of high voltage rectifiers. This will require pushing a button to start
the tube but is by far the simplest approach in terms of the number of
required parts. See the section: "Piezo impulse starting circuits".
4. Implement a wide compliant range constant current supply as described with
respect to the Linear Technology application note (AN-49, above). Until
the tube fires, the supply will attempt to increase the voltage (within
its compliance range) until current starts flowing. Also see: "Sam's
inverter driven HeNe power supplies".
For a basic AC line powered supply, I recommend the parasitic multiplier
approach (1) as it can be constructed from readily available inexpensive
components: 1N4007s (about 5 cents each) and .001 uF, 1,000 V ceramic disc
capacitors (about 10 cents each). Depending on the number of stages in the
multiplier, between 6 and 28 of each of these components will be required.
Of course, you can also use higher voltage diodes and capacitors to simplify
the construction but they will probably be much more costly. See the
section: "Sam's line powered HeNe laser power supply" for a tested design
using inexpensive parts.
VOLTAGE MULTIPLIER STARTING CIRCUITS
These are called 'parasitic multipliers' since they feeds off of the main
supply and are only really active during starting when no current is flowing
in the HeNe tube.
A voltage multiplier can be constructed from common diodes and capacitors
relatively easily. Alternatively, a 3X or 4X type may be purchased as a
replacement for the type found in some color TVs and monitors - you may even
have one gathering moss in your junk box! See the section: "Color TV or
monitor voltage multiplier".
A typical design with 7 diodes (3-1/2 stages) is shown below. For small
tubes, fewer stages can be used. Going much beyond n=7 or 8 (4 stages) is
probably not useful as the losses from diode and stray capacitance and leakage
will limit output.
R1 C1 C3 C5 C7
X o---/\/\---||------+-------||------+-------||------+-------||------+
1M, 1 W D1 | D2 D3 | D4 D5 | D6 D7 |
+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o HV+
| | | |
Y o----------+-------||------+-------||------+-------||------+
C2 C4 C6
HV- o-------------------------------------------------------------------o HV-
Where:
* X = Input. This is the pulsating output of the transformer.
* Y = Input. This is the DC output of the rectifier or doubler and filter.
* HV- = High Voltage return (this may not be system ground!)
(Note: all the power supply schematics in this document have these points
labeled consistently.)
With n diodes, HV(peak) is approximately (X(peak) * (n + 1)) + Y and
HV(average) is (X(peak) * n) + Y.
Note that an even number of diodes may be very slightly better since there is
less ripple on the output when the tube is running. However, with a high
value R1 (10M) this is so small anyway that it really should not matter
and an odd number of diodes saves components but results in nearly the same
peak starting voltage.
For use in HeNe laser starting applications where no real current is required,
R1 limits power to the multiplier once the tube fires. Power is then drawn
from point Y through the string of diodes.
Multipliers can be used with both line operated supplies and high frequency
inverters but since the capacitors must be larger at the (lower) line
frequency.
* Typical capacitor values for high frequency inverter: 100 pF.
* Typical capacitor values for line operated supply: 250 pF to .001 uF.
The voltage ratings of the diodes and capacitors must be greater than the
p-p output of the transformer.
Because the capacitors used in the multiplier are so small, they cannot really
supply much current. Once the tube fires and current starts to flow, the
ladder just becomes some series diodes. Then you're back to the basic power
supply output (rectifier or doubler and filter capacitors), with a few diodes
in series.
VOLTAGE MULTIPLIER OPERATION
Making the assumption that this circuit actually works, it is easiest to
understand its operation in the steady state with no load. (Trying to keep
track of each of the nodes on each cycle is virtually impossible without a
circuit analysis program.) Ideally, in the steady state, no current will be
flowing through any of the diodes.
There are two types of nodes:
* Nodes connected to the capacitors on the bottom rung of the ladder will
have filtered (constant) DC voltage on them.
* Nodes connected to the capacitors on the top rung of the ladder will have
pulsating DC voltage with the p-p voltage from X. With no losses, the
capacitors will pass the AC input waveform unchanged (in this simplified
analysis). Therefore, the waveform from X will be reproduced at each of
these nodes.
In order for no current to flow through the diodes:
- The peak positive value of each of these nodes must not exceed the DC
voltage on the node to its right.
- The peak negative value of each of these nodes must not exceed the DC
voltage on the node to its left.
It follows from this that the voltage (DC or average) must increase by a value
of V(peak) after each diode. Otherwise, current would be flowing on either
the negative or positive peak of the input.
Assuming a peak-to-peak amplitude of 2 units (just to keep the diagram
simple), the voltages at each node will be:
R1 C1 +1(AC) C3 +3(AC) C5 +5(AC) C7 +7(AC)
X o---/\/\---||------+-------||------+-------||------+-------||------+
+0 D1 | D2 +2 D3 | D4 +4 D5 | D6 +6 D7 |
+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+---o HV+
| | | 1|
Y o----------+-------||------+-------||------+-------||------+
C2 C4 C6
G o----------------------------------------------------------------------o HV-
Where:
* +N denotes a DC value of Y + n * V(peak).
* +N(AC) denotes a pulsating voltage with an average value of Y + n * V(peak)
and a p-p value of 2 * V(peak).
Thus, the nth diode's output has multiplied the peak value by n. The peak
value of HV+ will actually be Y + 8 * V(peak) or about 5 times the p-p output
of the transformer. (With one more diode and capacitor, this would be a DC
output of this same voltage.)
In practice the actual output will likely be somewhat less due to stray
capacitance and other losses.
COLOR TV OR MONITOR VOLTAGE MULTIPLIER
Some color TVs and computer or video monitors do not generate the high voltage
directly from the flyback transformer. Rather, they use a voltage multiplier
to step up 6 to 10 KVAC (from the flyback - no rectifier) to the 20 to 30 KVDC
or so required by the CRT. If you have a pile of dead TVs or monitors rotting
in your garage, at least one may have a suitable multiplier block. If not,
they are available for around $10 to $20 from electronics distributors dealing
in parts for the service industry (e.g., MCM Electronics, Dalbani, Premium
Parts, etc).
Since these voltage multiplier blocks are designed for horizontal deflection
(e.g., ~15.7 KHz), they are ideal for drive from an inverter. I have also
tested two different multipliers on my line powered (60 Hz) supply without any
problems. In fact, one of those I tried was removed from a TV because it had
a short in the focus divider network but this didn't affect HeNe tube starting
performance at all!
These are usually 3X units though some 4X types are also available. Hooking
them up is very straightforward and the entire multiplier is well insulated
(totally potted in Epoxy) with a high voltage output lead (just remove the CRT
suction cup connector). The unneeded terminals (e.g., F, CTL) can be ignored.
Some of these have a capacitively coupled drive input. If this is NOT the
case, the addition of a high voltage capacitor (.001 uF, 3 KV typical) in
between the X and IN terminals is required to achieve the full multiplication
factor. If you do not know if there is an internal capacitor, include C1 - it
will not hurt anything. Since you do not want much current from the voltage
multiplier, insert a 1M to 10M series resistor in this path as well.
HV Multiplier
R1 C1 +----------------+
X o----/\/\----------||--------| IN HV |----------o HV+
10M 1 W .001 uF, 3 KV | ECG535 F |-- NC
Y o----------------------------| DC CTL |-- NC
+----------------+
G o--------------------------------------------------------o HV-
ECG535 is just one typical 3X model. Almost any HV multiplier of this general
type will be suitable. However, if you have a choice, obtain a 4X multiplier
as this will provide a bit more margin (though a 3X model should be adequate
for most HeNe tubes). The IN terminal may be called GND, REF, or COM on some
models - or there may be a pair of terminals with two of these names. If so,
they should be tied together. F and CTL are Focus and Control respectively.
These and any other special terminals can be left unconnected.
STARTING CIRCUITS USING PULSE OR FLYBACK TRANSFORMERS
These approaches represent alternatives to the use of a voltage multiplier.
With the auto-trigger circuit, they are equally effective (hey, if it starts,
it starts!) and may be somewhat easier to construct since fewer parts are
involved. Some further simplification is possible if you are willing to
forgo the auto-trigger circuit and use a pushbutton switch instead of the SCR.
While the current required to start a HeNe tube is negligible, the energy
needed to achieve sufficient voltage given the stray capacitance of the
wiring, HeNe tube anode, and other connected components is not. With a
voltage multiplier type starting circuit, this just means it takes a little
time to charge up to the required voltage but as long as the leakage is small
(it usually can be ignored), the tube will start eventually. For a pulse
type staring circuit, the energy must be provided in one shot or a charge
pump must be used to accumulate smaller energy packets.
Thus, pulse trigger starting circuits are more effective if the wire length
(and thus the capacitance) between the power supply's final rectifier and HeNe
tube anode is minimized.
Locating a large enough pulse transformer to start the tube in one shot may
prove to be a challenge. However, a charge pump (really, just an extra high
voltage diode) can be added to any of these circuits so that the energy in
each pulse can be accumulated. The tiny trigger transformer from a disposable
pocket camera will not work very well by itself but should work with a such a
charge pump. See the section: "Pulse trigger circuit using small flyback
transformer" for the charge pump configuration.
A flyback transformer from a small B/W TV or computer terminal does work quite
nicely with a charge pump. An automotive ignition coil would probably be
large enough for single shot operation.
The first three circuits that follow differ only in the type of trigger
transformer used but operation is otherwise identical. These have not been
tested.
The final circuit uses a flyback transformer from a long forgotten video
display terminal and has been tested using a manual pushbutton.
Also see the section: "Piezo impulse starting circuits" for some super simple
alternatives.
PULSE TRIGGER CIRCUIT - TRIGGER TRANSFORMER WITH ISOLATED HV WINDING
This is the basic circuit where the primary and secondary of the trigger
transformer are isolated with an insulation breakdown rating of at least
12 KV. The major difference between this circuit and those to follow is
that it is referenced to the system common (HV-) rather than floating at
a high voltage (on one of the filter capacitors).
Operation is similar to a repeating strobe trigger.
Z o--------------------+-----------+
| | R5 o
/ R1 +---/\/\--+------+---------+ ||
\ 10M 4M 1W | | )|| +---o Y
/ NT1 | __|__ SCR1 )||(
| NE2 | _\_/_ TIC106 )||(
Q1 R8 | +--+ R3 T | / | )||(
MPSA43 +--/\/\--+---+--|oo|--/\/\--+--|---' | +-----+ ||(
| 100K | | +--+ 1K | | | | ||(
Tube- R8 |/ C / _|_ / / | _|_ C2 ||(
o-+--/\/\--| R2 \ --- C1 R4 \ \ R6 | --- 1 uF ||(
| 1K |\ E 1M / | .5 uF 1K / / 600K | | 600 V ||( CR1 HV+
/ | | | 200 V | | | | || +--|>|--o
\ R7 +--------+---+--------------+--+------+---+ o
/ 200 | T2
| |
o-+----------+
HV-
The voltage divider formed by R1 and R2 charges C1 from the high voltage power
supply (Z, a lower voltage tap if possible to reduce the dissipation in R1 and
R2). At the same time, C2 charges from R5 and R6 (this time constant is faster
than that of the relaxation oscillator). Once the voltage across NT1 (NE2
neon tube) reaches about 90 V, NT1 breaks down dumping C1's charge through
the gate of SCR1. This turns on and discharges C2 through T2 generating a 5
to 10 KV pulse in series with the high voltage power supply ionizing the gas
in the HeNe tube. CR1 must be a 15 KV or greater high voltage rectifier and
prevents reverse voltage from appearing at the tube.
Should the HeNe tube not fire on the first pulse, the process repeats at about
a 2 Hz rate until current starts flowing in the tube. A current of about 3.5
mA through the HeNe tube and R7 results in a voltage drop of .7 V across B-E
of Q1 turning it on. This short circuits the relaxation oscillator shutting
off the triggering circuit.
Component values can easily be adjusted to accommodate the specifications of
your specific power supply voltage and HeNe tube current.
These are all basically capacitive discharge ignition systems and indeed,
an automotive ignition coil may be satisfactory for the trigger transformer
where isolation is not needed!
ISOLATED PULSE TRIGGER CIRCUITS
The following two circuits are virtually identical. The only difference is
whether the dots on the pulse (trigger) transformer match or oppose. One of
these should be used when the trigger transformer shares a common lead between
the primary and secondary.
Operation for both is as follows:
The trigger capacitor, C2, charges through the voltage divider formed by R1 and
R2. When a pulse is input to the gate of the SCR via the high voltage coupling
capacitor, C1, it triggers dumping C2 through the primary of the trigger
transformer.
WARNING: The voltage rating on C1 must be adequate - with a safety margin - for
your power supply.
The input, T, comes from the autotrigger circuit which is part of the circuit
shown in the section: "Pulse trigger circuit - trigger transformer with
isolated HV winding".
* In this circuit, the primary and secondary of trigger transformer share a
common lead. Use this circuit where the dots on the windings match.
Y o----------------+-------+-----------+--------+
| | | T2 |
R1 \ __|__ SCR1 +-+ +-+
1M / _\_/_ TIC106 o )||( o
\ / | )||(
C1 | | | )||(
T o----||----|----+ | )||(
.01 uF | | | )||(
2 KV | R3 / | )||(
| 1K \ | +-+ ||(
| / | | ||(
| | | C2 | ||(
+----+--+------)|---+ ||(
| | 1 uF ||(
R2 / _|_ 600 V ||(
1M \ --- C3 ||(
/ | .01 uF ||(
| | 600 V ||( CR1
W o----------------+----+--+ +---|>|---o HV+
* In this circuit, the primary and secondary of trigger transformer share a
common lead. Use this circuit where the dots on the windings oppose.
Y o----------------+-------+---------------+--------+
| | | T2 |
| | +-+ +-+
| | o )||(
R1 \ _|_ C2 )||(
1M / --- 1 uF )||(
\ | 600 V )||(
| | )||(
| | +----+ ||(
| | | ||(
| | SCR1 __|__ ||(
| | TIC106 _\_/_ ||(
| | / | ||(
C1 | | | | ||(
T o----||----|-------|---------+ | ||(
.01 uF | | | | ||(
2 KV +-------+----+ R3 / | ||( o CR1
| | | 1K \ | +---|>|---o HV+
R2 / C3 _|_ | / |
1M \ .01 --- | | |
/ uF | +----+--+
| 600 V |
W o----------------+-------+
PULSE TRIGGER CIRCUIT USING SMALL FLYBACK TRANSFORMER
This circuit has only been tested with a manual push button. Usually, 3 or 4
presses of the button are needed to build up the needed starting voltage via
the charge pump formed by CR1 and CR2. These are needed because a single
press of the button may not generate enough charge to achieve the required
starting voltage. They enable the charge to be accumulated on the stray
capacitance of the wiring, CR1, and HeNe tube anode. The diodes should have a
PRV rating greater than the starting voltage requirement of your HeNe tube,
15,000 V should be sufficient. This type of diode is typically used in B/W
TVs and monochrome computer monitors. Microwave oven high voltage rectifiers
also work but not nearly as well (many more presses of the button are needed)
due probably to their higher capacitance and leakage.
TVs or monochrome computer monitors. Microwave oven high voltage rectifiers
should also work unless their leakage is too high.
Higher input voltage, more or fewer turns on the flyback, or a different
capacitor may improve response - your challenge!
The primaries on the flyback are ignored and a new one is added - 5 turns of
#20 or thicker insulated wire wound anywhere on the core where it will fit.
As long as the original primary windings are not shorted, they will not
interfere with circuit operation.
Locate the HV return and guess at the polarity - it will work properly only
one way since the output is a huge spike. Reverse the input connections if
you cannot get the tube to fire and you think everything else is correctly
wired.
I used a separate 50 VDC power supply to drive this circuit but you can use a
tap on the main filter capacitor of the main supply as well.
R1
+ o-------------/\/\---+------------+ +----o Y
1K | | T2 |
| | +--+---|>|---+
| | o ||( CR2 |
| +----+ ||( |
_|_ C2 )||( |
50 VDC --- 20 uF 5 T )||( |
| 100 V #20 )||( |
| )||( |
| +----+ ||( |
| S1 | ||( o | CR1
| _|_ | +------------+---|>|---o HV+
- o--------------------+-----o o----+
Start
An alternative to the pushbutton switch (or SCR circuits in the previous
designs) is a small low power inverter that is only active when starting.
Its output is then placed in series with the HV+ of the main supply as above
(Y to HV+).
See the various circuits in the document: Various Schematics and Diagrams
for ideas. The "Simple High Voltage Generator" will start a typical HeNe tube
using only 4 or 5 VDC for its power input.
PIEZO IMPULSE STARTING CIRCUITS
These circuits use a manual pushbutton but are about as simple as you can get.
They are based on the spark generating assembly from a disposable butane
lighter that uses a piezo element to generate the spark (as opposed to old
fashioned flint and wheel) or the starter assembly from a gas grill. These
simple devices are based on the piezo-electric effect which results when
certain materials are twisted or otherwise deformed. Even these dirt cheap
lighter or gas grill starters are able to produce more than 10,000 V
mechanically with the press of a button.
A simple charge pump using a pair of high voltage diodes completes the
starting circuit. These are needed because a single press of the button may
not generate enough charge to achieve the required starting voltage. They
enable the charge to be accumulated on the stray capacitance of the wiring,
CR2, PE1, and the HeNe tube anode. The diodes should have a PRV rating
greater than the starting voltage requirement of your HeNe tube, 15,000 V
should be sufficient. These type of diodes are typically used in B/W TVs or
monochrome computer monitors. Microwave oven high voltage rectifiers also
work but not nearly as well (many more presses of the button are needed) due
probably to their higher capacitance and leakage.
Both of the circuits below work but the second one may have an edge since the
capacitance being charged is slightly smaller since it doesn't include PE1
and its wiring:
HV+
o
CR1 CR2 | Rb
Y o------|>|----+----|>|----+----/\/\-------o Tube+
| |
| - + |
+----|X|----+
|--^ PE1
HV+
o
CR1 CR2 | Rb
Y o-----+----|>|----+----|>|----+----/\/\-------o Tube+
| |
| - + |
+----|X|----+
|--^ PE1
Complicated, huh? CAUTION: Since the piezo assembly/push button is connected
to the positive terminal of the high voltage power supply, make sure it is
well insulated or the only thing being triggered may not be the HeNe tube!
Make sure the wiring is short and well insulated - these high voltage pulses
tend to go wherever they want unless properly trained :-).
I tested this with the piezo assembly removed from a Scripto lighter found
in the park. (You will have to agree that this is classic McGyver!) The
required part is self contained and pops off with minimal disassembly. With
this particular device, the pointy electrode is -. I do not know if this is
always the case. If the tube does not start after a reasonable number of
button presses, try interchanging the connections to the piezo assembly.
Replacement push button starters for gas grills can be purchased at most home
centers and may even work better because they are likely to produce a higher
voltage higher energy spark. I will not be responsible if your dad cannot
barbecue the dogs and burgers because you 'borrowed' the starter :-).
As with transformer based pulse trigger starting circuits, these are more
effective if the wire length (and thus the capacitance) between the first
high voltage rectifier (CR1) and the HeNe tube anode is minimized.
For my bare 1 mW HeNe tube, two presses of the button were required. For a
laser head with a 3 foot long high voltage cable, 4 or 5 presses were needed
but it always started eventually.
CURRENT REGULATORS FOR HeNe POWER SUPPLIES
Since for optimal performance, the HeNe tube expects a specific operating
current, a regulator is desirable but not essential. Current is not nearly
as critical with HeNe lasers as with laser diodes. A few percent on either
side of the recommended value won't matter.
Supply voltage fluctuations do affect operating current proportionately more
than would be expected since the voltage drop across the tube is fairly
constant. Therefore, nearly the entire change appears across the ballast
resistor. The smaller the ballast resistor, the larger the current change
for a given voltage change. This may mean that a 2 percent change in line
voltage produces a 10 percent change in tube current - still not a big deal
for most applications.
However, where stability is required (both short term and long term due to
component and tube aging) or where it is desired to be able to switch tubes
without monitoring tube current and adjusting the input voltage, a regulator
is essential.
Regulators can be linear in the high-side (anode circuit) or low-side (cathode
circuit). Both types are used though I see little reason to consider a high
side regulator since it must also be insulated for the starting voltage. Also,
it is particularly easy to add modulation capability to a low-side regulator.
In the case of inverter type power supplies, there is also the option of
controlling the switching frequency or pulse width via a feedback circuit from
secondary to primary - a true regulated switching power supply.
In addition to the circuits below, see the chapter: "Complete Helium Neon
Laser Power Supply Schematics" for additional possibilities.
LOW-SIDE POST-REGULATORS
This circuit (similar to the one described in the section: "Spectra Physics
model 155 HeNe power supply") can be used to stabilize the output of an
unregulated supply. Within its compliance range, the current will be
approximately: 10.3 V/R2.
+------------+---o Tube-
| |
R1 |/ C Q1 |
Z o----------/\/\------+----| MJE3439 _|_, VT2
5M | |\ E '/_\ 150 V
1 W | | |
_|_, / |
VR1 '/_\ +->\ R2 _|_, VR3
1N5241B | |v / 5 K '/_\ 150 V
11 V | | \ |
| | | |
+---+--+------------+---o HV-
Caution: If your tube is mounted with its cathode connected to a metal case,
earth ground must be tied to Tube- for safety as Tube- may be at a potential
of several hundred volts with respect to HV-.
The compliance range is about 300 V so a ballast resistor still needs to be
selected to enable the regulation to be effective. Use the procedure described
in the section: "Identifying the range of a power supply and selecting the
ballast resistor" to determine an appropriate ballast resistor value for the
desired tube current and a power supply voltage 150 V less than that of your
(unregulated) supply. This circuit should then maintain that current constant
and permit some adjustment (or modulation if desired) of the current on either
side of the set-point.
For greater compliance, a cascade of high voltage transistors can be used
as outlined in the section: "High compliance low-side regulator". Also
see the section: "Spectra Physics model 247 HeNe power supply" which is a
high-side version of a similar circuit.
HIGH COMPLIANCE CASCADE REGULATORS
The circuit below is suitable for providing current regulation for high power
HeNe power supplies. As drawn it should be capable of about 1,000 to 1,200 V
compliance at up to 10 mA. Additional high voltage NPN transistors and base
resistors may be added to extend its range still further.
Rb Tube+ +------------+ Tube- Rs IL2 LED G
HV+ o---/\/\--------|- |-|----+---/\/\----|>|------+----+
+------------+ _|_ 1K Beam On | |
LT1 - R1 / |
120K \ |
2 W / |
| |/ C Q1
+--| MJE2360T
| |\ E
R2 / |
120K \ |
2 W / |
| |/ C Q2
+--| MJE2360T
| |\ E
R3 / |
120K \ |
2 W / |
R4 | |/ C Q3
=-----/\/\------+--| MJE2360T
| 120K |\ E
| 2 W |
| |
| |
Rx | R5 |/ C Q4
Z o------------/\/\----------+-----+-----/\/\------+--| MJE2360T
470K | | 10K | |\ E
| R6 / |/ E |
|100K \<------------| |
CR1 _|_, / Adjust Q5 |\ C |
1N4744 '/_\ | 2N3906 | |
15V | | (PNP) +----+
| | +--+ |
| | | v R7 R8 |
HV- o---+-----+---+-/\/\----/\/\---+
Range 5K 1.5K
The PNP transistor (Q5) buffers the reference voltage so that the very low
current source (point Z through Rx tapped off of the main filter capacitor
string) can drive the base of the cascade. If the reference can supply a
couple of mA, Q5 can be omitted.
The base resistors, R1 through R4 equally distribute the voltage between
G and HV-. The respective transistors act as emitter followers and maintain
approximately the same voltages across their C-E terminals. Within the
compliance range, the voltage across R7+R8 will be equal to the voltage
on the wiper of R6 (give or take a diode drop depending on whether Q5 is
present).
Caution: Make sure that the open circuit voltage of your power supply minus
the laser tube and ballast resistor voltages cannot exceed the total breakdown
voltage of the transistor cascade (4 * Vceo in this case as bad things may
happen)! As drawn, it is certainly suitable for a 2,200 V power supply with
almost any tube. However, if you are using this with a 3,000 V supply and
a .5 mW tube requiring 1,100 V at 3 mA, the compliance range will be exceeded
and the transistors may blow - pop-pop-pop-pop! Then, you get maximum current
through the ballast resistor and laser tube which will make them extremely
unhappy :-(. A bag of 200 V zeners in series could be used to limit the
voltage across the transistor cascade.
R7 sets the overall current range. R6 adjusts the current from a minimum
determined by the maximum voltage across R1 through R4 to the limit set by R7.
However, regulation should be better when R6 is set near its maximum - it can
be omitted entirely if desired and R7 used as the current adjust. The only
thing undesirable about this is that this control is somewhat non-linear
so that the high current portion is all squashed at one end.
Rs is a current sense resistor. Monitoring across it with a volt meter is
a convenient way of setting tube current. Sensitivity is 1 V/mA. A current
meter can also be put across it and will read tube current directly. The LED
provides a 'Beam On' indication and rough measure of tube current as well.
MODULATION OF HeNe LASERS
Impressing an audio or digital signal on the light beam from a HeNe laser can
be accomplished in a number of ways:
* Electronically by varying the HeNe tube current. However, this will not
permit a 100 % variation from full on to full off. Depending on the
particular power supply/ballast resistor/HeNe tube combination, going below
a threshold - perhaps 50% - will result in the tube going out and needind
to be restarted. This will depend on combination of the power supply
voltage, ballast resistor, and tube characteristics.
The sensitivity of optical output power to changes in tube current is also
not 1:1. Well below the optimal current specification, it is more like
1:2 to 1:5. Near the peak optical output, this may exceed 1:10 and at some
point results in negative modulation as increasing tube current actually
reduces optical output.
Modulating a HeNe laser beam by controlling the input to the a commercial
power supply will probably not work due to the filtering in the power supply
output and possible regulation against just such input voltage variations.
However, if you have built one yourself, at least *adjusting* power level in
this way is probably possible though any output filtering will severely limit
frequency response.
Usually, electronic modulation is accomplished by adding a circuit to the
return (cathode circuit) of the HeNe tube to control tube current. In this
respect, regulators and modulators may be combined.
* Mechanically by introducing a blocker into the beam path in such a way that
the position of an actuator like a galvo, loudspeaker, or piezo element,
affects amount of light that emerges.
* Electro-optically by adding a spatial light modulator or acousto-optic
deflector to control beam intensity. Some of these devices are too costly
for general hobbyist use but you might have salvaged one from an old laser
printer.
Electronic modulation techniques are covered in the remainder of this chapter.
They are easy to implement and inexpensive.
However, mechanical and electro-optic approaches do have their advantages
including a potentially much wider modulation intensity range (from full off
to full on, which, as noted, is not really possible with electronic techniques
controlling tube current). They also do not require any modifications to the
HeNe laser power supply and are thus necessary if this is not possible or
desirable.
ELECTRONIC MODULATION OF HeNe LASERS
The basic techniques are all similar and modulate optical output power by
controlling tube current. These are often combined with the current regulator
or provided as an addition to it or used on a power supply without regulation.
(They cannot be used in addition to regulation as the modulator and regulator
will fight one-another.)
Several suggestions for implementing power supply output side modulation are
outlined below. However, the following should be kept in mind:
* There is a limited range over which HeNe tubes operate properly. Too low
and the discharge will go out; too high and aside from excessive heating,
the optical output will actually *decrease*. For example, a tube rated
for 4 mA may actually only be usable within a 3 to 5 mA range. The low
limit depends to some extent on the power supply voltage and ballast
resistor value - a higher voltage (and ballast resistor) will permit the
tube to be stable at a lower current.
* The relationship between tube current and optical output power within the
range over which the discharge is stable is not very high gain or linear.
In other words, a large change in current will only result in a small
change in optical output - you won't get anywhere near 100% modulation.
* As a result of the output power decreasing with current above optimal, the
phase of the modulated signal will invert and driving it on either side
of optimal current will result in a full wave rectification effect and
serious distortion.
Thus, the peak current provided by these circuits should be set less than
or equal to the maximum operating current for your tube. This involves
selection of the ballast resistor and power supply and/or regulator current
set-point.
Where wide range (approaching 100 percent) modulation is required, external
electro optical or electro mechanical modulation may be a more effective
solution.
Depending on the type of information you would like to transmit over the beam
and its bandwidth or data rate, dual tone or some kind of frequency or phase
modulation may be better than simple amplitude modulation.
Note: These circuits have not been fully tested. Some tweaking may be
necessary.
MODULATION USING AN AUDIO OR POWER TRANSFORMER
Add a small audio transformer or low voltage power transformer (in reverse)
with its primary (high impedance/high voltage side) in the return (cathode
end) of the tube and drive its primary from an audio amp or whatever you
like. You probably want a couple hundred volts at full amplitude. Just
don't drive it to the point of the tube turning off.
HeNe tube
Rb Tube+ +------------+ Tube-
Laser PS+ o-------/\/\----------|- |-|-------+----+
+------------+ | _|_ (see note
| - below.)
+---+
||(
o---------+ ||(
)||(
Input from LV )||( HV
audio amp )||(
)||( Small audio or
o---------+ ||( power transformer
||(
+---+
|
Laser PS- o------------------------------------------+
Caution: If your tube is mounted with its cathode connected to a metal case
as is the case with typical commercial laser heads, earth ground must be tied
to Tube- for safety as Tube- may be at a potential of several hundred volts
with respect to HV-. If the tube is enclosed and insulated from the user,
this is not necessary.
MODULATION USING A HIGH VOLTAGE NPN TRANSISTOR
Put a high voltage transistor (e.g., MPSW42) in the return (cathode end)
with a pair of 120 V zener diodes in series across it to prevent the voltage
from going above 240 V. Bias the transistor to obtain linear control of
tube current. Then, drive the transistor from your audio signal. This
is basically a voltage controlled current source.
The circuit below is designed for a 4 mA operating current. With R3 set at
200 ohms, a 1 V p-p modulation input will vary the tube current from 1.5 to
6.5 mA. However, the range of your tube may be much more restricted than this
depending on your tube, power supply voltage, and ballast resistor value.
HeNe tube
Rb Tube+ +------------+ Tube-
Laser PS+ o-------/\/\----------|- |-|-------+
+------------+ |
R1 |
+---/\/\---+-------+
| 100K | |
C1 10 uF | |/ C |
+ o------)|----+--------| Q1 _|_,
- + | |\ E '/_\ ZD1
Line level | MPSW42 | | 120 V
audio / / ^ _|_,
R2 \ R3 \<-+ '/_\ ZD2
- o 1.5K / 500 / | | 120 V
| | | | |
Laser PS- o------------------+------------+----------+--+----+
Caution: Do not use this circuit with a tube whose metal housing is not
totally insulated from the user since the cathode will have a potential of
several hundred volts PS- (and the signal return). For such tubes, see the
alternative circuit below and the section: "Enhancements to Aerotech model PS1
HeNe power supply" for some possibilities.
The use of a transistor like an MJE3439 or MJE2360 would permit a wider
range of control but 200 V is likely to be sufficient. One of these would
also be needed for higher current tubes since the MPSW42 is only rated at
about 1 W (100 V * 4 mA is .4 W). A vacuum tube triode or high voltage
FET in a similar configuration could even be used. The tube would be kind
of quaint :-). See the secton: "Vacuum tube regulator/modulator" for
details.
The Vceo rating for such a transistor may need to be several hundred volts
depending on your HeNe tube and power supply, ballast resistor value, and
how much modulation is desired. The voltage divider formed by R1 and R2
and emitter resistor R3 set the operating current and voltage across the
transistor with no signal.
A small HeNe tube will have perhaps 1,200 V across it and 300 V across the
ballast resistor at the recommended operating current. The voltage across
the tube (a negative resistance discharge device), will be more or less
constant around the operating current (but may increase as the current is
reduced). It is the 300 V that you have to play with. If the transistor
drops 150 V, the ballast resistor will only drop 150 V and the current will
be cut in half.
The compliance range for the circuit shown is about 200 V so a ballast
resistor still needs to be selected to enable the regulation to be effective
from 0 to maximum modulation.
Use the procedure described in the section: "Identifying the range of a
power supply and selecting the ballast resistor" to determine an appropriate
ballast resistor value for the desired tube current and a power supply
voltage 100 V less than that of your (unregulated) supply. This circuit
should then maintain that current constant with no audio input and permit
modulation of the current on either side of the set-point up to a change of
approximately +/-100/Rb mA.
MODULATION USING A HIGH VOLTAGE PNP TRANSISTOR
The following circuit permits the cathode to be at earth and signal ground
potential so that metal cased laser heads can be modulated without fear of
electrical shock:
HeNe tube
Rb Tube+ +------------+ Tube-
Laser PS+ o-------/\/\----------|- |-|-------+----+
+------------+ | _|_
| -
+ o------------+----------+--+----+
| | | |
/ / | |
Line level R2 \ R3 \<-+ _|_,
audio 1.5K / 500 / v '/_\ ZD1
| | | 120 V
C1 10 uF | |/ E |
- o------|(----+--------| Q1 _|_,
+ - | |\ C '/_\ ZD2
R1 / MPSW92 | | 120 V
100K \ (PNP) | |
/ | |
| | |
Laser PS- o-------------------------------+----------+-------+
This circuit uses a PNP transistor to permit the signal input to be near the
laser tube cathode and earth ground. Otherwise, it operates identically
to the previous one.
MODULATION USING A VACUUM TUBE
Transistors like the MJE3439, MJE2360T, or even the MPSW92 or MPSW42 are
generally adequate for these tasks. A high voltage MOSFET can also be used.
However, if you long for the warm quaint glow of a vacuum tube, a similar
approach is easily implemented with common (well they used to be common)
miniature receiving tubes. In the tube's defense, it may be more difficult
to fry tubes than semiconductors!
If I recall anything about tubes (which was a long time ago) select a cathode
resistor to bias the grid negative to set the laser tube operating current
(less than its maximum) and then drive the grid from a voltage source like
the speaker output of an audio amplifier. The voltage on the plate will then
vary resulting in an incremental current change of roughly: delta(Vin)/TC
where TC is the transconductance of the vacuum tube at its operating point.
For example, using a 12BH7A medium mu twin triode (you can use the other half
for a fabulous audio preamp!), the grid voltage will need to be in the range
-0 to -30 V for a compliance range of 400 V. (Estimated from the RCA Receiving
Tube Manual, 1970?). I just picked a triode with a suitable maximum plate
voltage rating. Many many other tube types (likely cheaper also - the 12BH7A
seems to be quite expensive for some reason) will work as well. At a plate
voltage of 250 V, TC is 3,100 umhos resulting in an overall sensitivity of
about 3.1 mA/V. Thus, this circuit would work quite nicely with line level
audio signals since a total variation of only 1 to 3 mA should be needed to
modulate a typical HeNe laser tube.
In the circuit below, R2 and R3 set the bias point depending on the current
requirements of your HeNe tube, power supply output voltage, and ballast
resistor. C2 is the cathode bypass capacitor (low frequency roll off of
around 20 Hz at the minimum (higher current) setting of R3).
HeNe tube
Rb Tube+ +------------+ Tube-
Laser PS+ o-----/\/\--------|- |-|------+----+
+------------+ | _|_
|P -
C1 ---
Signal in o------||------+------------------- - - - G ET1
.1 uF | .---. K 1/2 12BH7A
500 V | +------+--+ ^
| | | | |
R1 / | R2 / F F
1M \ | 470 \ (6.3 VAC or 12.6 VAC
/ C2 +_|_ / depending on connections)
| 10 uF --- |
| 100 V - | R3 /
| | 15K \<-+
| | / |
| | | |
Laser PS- o--------------+----------+------+--+
Caution: If your tube is mounted with its cathode connected to a metal case
as is the case with typical commercial laser heads, earth ground must be tied
to Tube- for safety as Tube- may be at a potential of several hundred volts
with respect to HV-. If the tube is enclosed and insulated from the user,
this is not necessary.
PULSING A HeNe LASER
Unlike solid state (e.g., ruby, Nd:YAG) lasers which produce extremely intense
short pulses of light, HeNe lasers really are continuous wave devices. It is
not possible to obtain high peak power from a HeNe laser by driving it harder,
Q-switching, or other exotic techniques.
A high voltage power supply that produces short pulses exceeding the tube's
starting requirements will result in output pulses of light. With appropriate
design, pulse duration and repetition rate can be controlled. Depending on
needs, a flyback transformer or automotive ignition coil may be suitable. The
stability of the beam on such short times scales may be questionable, however.
High repetition rate pulsing may result in shortened tube life as well.
As with modulation, mechanical or electro-optic techniques are also possible.
A simple chopper wheel in the beam path may be perfectly adequate for your
needs and a lot less complex and costly than high tech alternatives!
HeNe LASER POWER SUPPLY SCHEMATICS
AC LINE OPERATED VS. INVERTER BASED POWER SCHEMATICS
A variety of techniques can be used to provide the starting and operating
voltages for HeNe lasers. It is, after all, just a special type of gas
discharge tube so almost any approach that can convince the HeNe tube to pass
the proper current will be satisfactory.
* AC line operated approaches are generally larger and heavier due to the
power transformer which must run at the line frequency - 50 or 60 Hz.
The designs are very straightforward and aside from the power transformer,
all parts are inexpensive and readily available.
* Inverter based approaches are generally small, light weight, and efficient,
but may require more sophistication in design and hard-to-obtain, expensive,
or custom parts.
These may run off of low voltage DC or the AC line but in the latter case
convert the AC into DC first and then use a high frequency chopper and small
transformer to generate their output.
Both types may include internal current regulation or have their voltage and
current adjusted with a variable power input (Variac or variable DC supply
as appropriate).
Modulation inputs may also be provided to permit the transmission of audio or
data over the HeNe beam to enable external closed loop control of beam power.
This chapter provides a variety of circuits of both types for the basic power
supply, some with regulators, modulation inputs, and other enhancements.
AC LINE OPERATED POWER SUPPLY SCHEMATICS
The first 5 circuits described in the following sections were reverse
engineered from commercial HeNe power supplies. There may be errors in
transcription as well as interpretation. In many cases, the transformer
secondary voltage was not marked and where the actual hardware was not
available for testing, an estimate of its value was made.
Many of these designs are quite old since modern commercial units tend toward
inverter designs since they can be more compact and have higher efficiency.
Unfortunately, these are nearly always potted in Epoxy and impossible to
analyze.
The last one I built using the scrounged power transformer from a long dead
and cannibalized tube type TV and a pair of high voltage capacitors (for the
main filter on the doubler) that had been sitting in a box minding their own
business for the last 15 years. My total cost for the remaining components
was about $5.
AC Line operated power supplies will drive HeNe tubes just as well as fancy
inverters and are somewhat easier to construct and troubleshoot.
The line side circuitry is not shown for any of these. See the section: "AC
input circuitry for HeNe power supplies" for details.
EDMUND SCIENTIFIC HeNe POWER SUPPLY
There were some inconsistencies in the component values of this circuit when
I first saw it. I have adjusted the RMS value of the transformer down from
710 to 650 VRMS so that the numbers work out closer to what one would expect.
Estimated specifications (Edmund Scientific):
Operating voltage: 1,700 V.
Operating current: 4 mA.
Starting voltage: around 5,300 V.
Compliance range: NA - no regulation.
(Portions from: Steve Nosko (q10706@email.mot.com)).
This is the power supply I traced out and measured which is in an Edmund
Scientific 0.5 mw. Laser circa probably around 1975. I bought a 1 mW. tube
(1986) when the old one broke. It is still running just fine. I think it
is a rather clever design and I don't think they come any simpler.
X C5 C7
+-------------------||-----------+----------||-----------+---o HV+
| D7 D8 | D9 D10 D11 D12 | R5
| +--|>|-|>|--+--|>|-|>|--+--|>|-|>|--+--/\/\--+
| D1 D2 D3 Y | C6 | 18K |
+---+--|>|-|>|-|>|--+----+----------||-----------+ 1 W / R6
||( | | | \ 33K
||( | C1 +_|_ / R1 / 1 W
||( | 4.7 uF --- \ 1M |
||( | 450 V - | / / R7
||( | | | \ 33K
||( | +----+ W Transformer: 650 VRMS, 20 mA / 1 W
||( | | | (primary not shown) |
||( | C2 +_|_ / R2 / R8
||( | 4.7 uF --- \ 1M \ 33K
||( | 450 V - | / / 1 W
||( T | | | D1-D7: 1N4007 or similar |
+-------------------+----+ / R9
| | | \ 33K
| C3 +_|_ / R3 C1-C4: 4.7 uF, 450 V / 1 W
| 4.7 uF --- \ 1M C5-C7: .001 uF, 2 KV |Tube+
| 450 V - | / .-|-.
| | | R1-R4: 1M, 1 W | | |
| +----+ Z R5-R9: (ballast, 18K+4x33K, 1W) | |
| | | LT1 | |
| C4 +_|_ \ R4 | |
| 4.7 uF --- / 1M ||_||
| 450 V - | \ '-|-'
| | | |Tube-
+--|<|-|<|-|<|--+----+--------------------------------------------+---o
D4 D5 D6 _|_ HV-
-
Note that there are no equalizing resistors across the 1N4007s. While I have
been building similar supplies without them, the use of 10M resistors across
each diode to equalize the voltage drops is recommended.
The 650 V transformer output feeds a voltage doubler (D1 to D6 and C1 to C4)
resulting in about 1,750 V across all the electrolytics. (Slightly less than
2 times the peak value of 650 VRMS.)
D7 to D12 and C5 to C7 form a classical voltage multiplier ladder which
generates a peak of up to 4 * V(peak) + 2 * V(peak) or 6 * 880 = 5,300 V.
This seems somewhat low but the power supply is for only a .5 mW tube. See
the section: "Voltage multiplier starting circuit" for a description of its
design and operation.
The 150K ballast resistor is actually constructed from 4 - 33K resistors
and one 18K resistor in series. It doesn't have to be, but this is convenient
and allows the ballast to be changed easily (or just tap off the appropriate
point for your tube. My notes show 600 V across the ballast resistor-combo.
The ballast resistor should be located close to the tube with as short a lead
as possible and as little capacitance to surroundings as possible. The tube
needs to see a high impedance source. This isn't super critical, but keep the
wire down to 1 to 3 inches and the first few resistors away from any case or
ground material.
Since there is no active regulator, the tube current will depend on the power
line voltage and other factors like temperature. However, the relatively large
ballast resistor in this power supply should minimize excessive variation.
SPECTRA PHYSICS MODEL 155 HeNe POWER SUPPLY
This is the schematic for another simple line operated power supply. This
one includes a current regulator which can easily be modified for any typical
tube requirement. It can also be converted to a modulator in a number of ways.
Estimated specifications (Spectra Physics 155):
Operating voltage: 1,700 V.
Operating current: 3.75 mA.
Starting voltage: greater than 10,000 V.
Compliance range: 1,400 to 1,700 V at top of ballast resistor.
High voltage diodes and capacitors are used in this design. An alternative
is to use inexpensive 6 - 1,000 V diodes for each 6 KV diode shown here, and
to use 6 - .003 uF, 1 KV capacitors in series for each 6 KV capacitor. I
would recommend 10 M ohm equalizing resistors across each lower voltage
device though for the diodes at least, this appears not to be essential.
X C103
+--------------||-------------+
| C100 | C101
+--------------||-------------+--------||---------+---o HV+
| CR101 | CR102 CR103 | R107
| +---|>|---+---|>|---+---|>|---+---/\/\---+
T100 | CR100 Y | C102 | 33K |
+---+-----|>|-----+-----+---------||--------+ 2 W |
||( | | |
||( C103 +_|_ / R100 |Tube+
||( 10 uF --- \ 470K T100: 1,300 VRMS, 20 mA .-|-.
||( 450 V - | / 1 W (primary not shown) | | |
||( | | | |
||( +-----+ W CR100-CR103: LMS60 (6 KV) | |
||( | | | | LT100
||( C104 +_|_ / R101 C100-C103: 560 pF, 6 KV | |
||( 10 uF --- \ 470K C103-C105: 10 uF, 450 V | |
||( 450 V - | / 1 W ||_||
||( T | | R100-R103: 470K, 1 W '-|-'
+---+ +-----+ R107 (ballast): 33 K, 2 W |Tube-
| | | |
| C105 +_|_ / R102 |
| 10 uF --- \ 470K +------------------+---o HV-
| 450 V - | / 1 W | _|_
| | | R103 |/ C Q100 -
| +-----+----/\/\------+----| MJE3439
| | Z 430K | |\ E
| C106 +_|_ 1 W | |
| 10 uF --- _|_, /
| 450 V - | CR104 '/_\ \ R106
| | 1N5241B | / 2.74 K
| | | \
| | | |
+-------------+--------------------+------+
The 1,300 V transformer output feeds a half wave rectifier (CR100) and filter
resulting in about 1,750 V across all the electrolytics. (Slightly less than
the peak value of 1,300 VRMS.)
CR101 to CR103 and C100 to C103 form a classical voltage multiplier ladder
which generates a peak of up to 4 * V(peak) + 2 * V(peak) or 6 * 1,750 = 10,600
but losses in the diode-capacitor network probably reduce this somewhat. See
the section: "Voltage multiplier starting circuit" for a description of its
design and operation.
Q100, CR104, and R106 form a constant current regulator which will attempt
to maintain the tube current at (Vz - .7)/R106 or about 3.75 mA in this case.
Its compliance range is about 300 V. This can easily be adapted to your
requirements by either changing CR104 or R106 appropriately.
SPECTRA PHYSICS MODEL 247 HeNe POWER SUPPLY
This one appears to be capable of driving higher power tubes and to have a bit
more sophisticated constant current regulator with wider compliance than the
Model 155. The regulator is in the positive feed instead of the return but
otherwise, the basic power supply design is similar.
Estimated specifications (Spectra Physics 247):
Operating voltage: 3,200 V.
Operating current: 2.3 to 10 mA.
Starting voltage: greater than 10,000 V.
Compliance range: 2,200 to 3,200 V at top of ballast resistor.
X R1 C1 C11
+---/\/\------||----------+---------||--------+
| 680K CR3 | CR4 CR5 | CR6
| +---|>|----+---|>|---+---|>|---+---|>|---+
T1 | CR1 Y | C10 | C12 |
+---+---|>|---+----+---------||---------+-----||-----+------+----+---o HV+
||( | | | | | |
||( | C2 +_|_ / R2 | R11 / |
||( | 10 uF --- \ 680K T1: 1,200 VRMS, 20 mA | 120K \ |
||( | 500 V - | / 1 W (primary not shown) | 2 W / |
||( | | | | | |/ C Q1
||( | +----+ W CR1-CR6: SCM60 (6 KV | +--| MJE3439
||( | | | | | |\ E
||( | C3 +_|_ / R3 C2-C9: 10 uF, 500 V | R12 / |
||( | 10 uF --- \ 680K C1, C10-C13: 500 pF, 6 KV | 120K \ |
||( | 500 V - | / 1 W | 2 W / |
||( | | | R2-R9: 680K, 1 W | | |/ C Q2
||( | +----+ R11-R14: 120K, 2 W | +--| MJE3439
||( | | | | | |\ E
||( | C4 +_|_ / R4 Q1-Q4: MJE3439 | R13 / |
||( | 10 uF --- \ 680K | 120K \ |
||( | 500 V - | / 1 W | 2 W / |
||( | | | | | |/ C Q3
||( | +----+ +------------------+ +--| MJE3439
||( | | | | | |\ E
||( | C5 +_|_ / R5 | R14 / |
||( | 10 uF --- \ 680K | 120K \ |
||( | 500 V - | / 1 W | 2 W / |
||( T | | | | R10 48K | |/ C Q4
+---|---------+----+ | +----/\/\----+--| MJE3439
| | | | | | |\ E
| C6 +_|_ / R6 | | |/ E |
| 10 uF --- \ 680K | +----------| |
| 500 V - | / 1 W | | Q5 |\ C |
| | | | CR7 _|_, 2N5086 | |
| +----+ C16 _|_ 1N5245A '/_\ +----+
| | | .047 uF --- | |
| C7 +_|_ / R7 6 KV | | R17 R16 |
| 10 uF --- \ 680K | Adjust +---/\/\---/\/\---+
| 500 V - | / 1 W | | | 5K 1.5K
| | | | +----+
| +----+ | | R15 R18
| | | | +---/\/\---/\/\---+
| C8 +_|_ / R8 | 20K 20K |Tube+
| 10 uF --- \ 680K | 2 W 2 W .-|-.
| 500 V - | / 1 W | | | |
| | | | | |
| +----+ Z C15 _|_ | | LT1
| | | .047 uF --- | |
| C9 +_|_ / R9 6 KV | | |
| 10 uF --- \ 680K | ||_||
| 500 V - | / 1 W | '-|-'
| | | | |Tube-
+---|<|---+----+--------------+------------------------------+---o HV-
CR2 _|_
-
The 1,200 V transformer output feeds a voltage doubler consisting of rectifiers
CR1 and CR2 and filter capacitors C2 through C9 resulting in about 3,200 V
across all the electrolytics. (Slightly less than 2 times the peak value of
1,200 VRMS.)
CR3 to CR6 and C1 and C10 through C12 form a classical voltage multiplier
ladder which generates a peak output of 4 * V(peak) + 2 * V(peak) or
6 * 1,700 = 10,200 V but losses in the diode-capacitor network probably
reduce this slightly. See the section: "Voltage multiplier starting circuit"
for a description of its design and operation.
C15 and C16 provide some additional filtering to the output so unlike the
previous supplies whose outputs include the last multiplier diodes without
filtering, this one is more pure DC. This would be better for laser
communications, for example, as the tube current will have less ripple.
Q1 through Q5, their associated resistors, and CR7 (15 V zener) maintains a
constant voltage of 15 V across the combination of R16 + R17 so the tube
current will be 15/(R16 + R17). For example, with the R17 set for 1.5 K, the
tube current will be 5 mA. The adjustment range is approximately 2.3 to 10 mA.
The voltage compliance range of this power supply should be over 1,000 V.
AEROTECH MODEL PS1 HeNe POWER SUPPLY
This one appears to be suitable for higher power tubes but is running at
very conservative voltage levels with the transformer that is provided. It
uses low-side regulation with a fixed output of about 2,000 V at 4 mA.
(Model number PS1 is arbitrary - supply was unmarked).
Estimated specifications (Aerotech PS1):
Operating voltage: 2,000 V.
Operating current: 4 mA.
Starting voltage: nearly 10,000 V.
Compliance range: 1,500 to 2,000 V at top of ballast resistor.
X R9 C9 C11 C13 C15
+---/\/\----||-----+-------||------+-------||------+-------||------+
| 100K, 1 W CR3 | CR4 CR5 | CR6 CR7 | CR8 CR9 | HV+
| +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
T1 | CR1 |Y | | | |
+---+---|>|---+----+----||----+-------||------+-------||------+ |
||( | | | C10 C12 C14 |
||( | C1 +_|_ / R1 |
||( | 10 uF --- \ 510K T1: 750 VRMS, 20 mA |
||( | 450 V - | / 1 W (primary not shown) |
||( | | | |
||( | +----+ CR1-CR9: (3 KV) |
||( | | | R10 /
||( | C2 +_|_ / R2 C1-C8: 10 uF, 450 V 47K \
||( | 10 uF --- \ 510K C9-C15: .005 uF, 3 KV 5 W /
||( | 450 V - | / 1 W \
||( | | | R1-R8: 510K |
||( | +----+ |
||( | | | |
||( | C3 +_|_ / R3 +--+
||( | 10 uF --- \ 510K |
||( | 450 V - | / 1 W |Tube+
||( | | | .-|-.
||( | +----+ | | |
||( | | | | |
||( | C4 +_|_ / R4 | |
||( | 10 uF --- \ 510K | | LT1
||( | 450 V - | / 1 W | |
||( T | | | | |
+---|---------+----+ | |
| | | ||_||
| C5 +_|_ / R5 '-|-'
| 10 uF --- \ 510K |Tube-
| 450 V - | / 1 W |
| | | +----+
| +----+ | _|_
| | | | -
| C6 +_|_ / R6 |
| 10 uF --- \ 510K |
| 450 V - | / 1 W |
| | | |
| +----+ |
| | | +----+
| C7 +_|_ / R7 MJE2360T | C |
| 10 uF --- \ 510K |/ |
| 450 V - | / 1 W +-----------| Q1 |
| | | R8 | |\ |
| +----+-------------/\/\-----------+ | E |
| | Z 470K | R11 / / R12
| C8 +_|_ 1 W ZD1 _|_, 3.6K \ \ 375 K
| 10 uF --- 1N4744 '/_\ / / 2 W
| 450 V - | 15 V | | |
| | | | |
+---|<|---+---------------------------------+-------------+----+---o HV-
CR2
Note: the laser head itself may have an additional ballast resistor (not
shown).
The 750 V transformer output feeds a voltage doubler consisting of rectifiers
CR1 and CR2 and filter capacitors C1 through C8 resulting in about 2,000 V
across all the electrolytics. (Slightly less than 2 times the peak value of
750 VRMS.)
CR3 to CR9 and C9 through C15 form a classical voltage multiplier ladder
which generates a peak of 2 * V(peak) + 8 * V(peak) or 10 * 1,000 = 10,000 V
but losses in the diode-capacitor network probably reduce this somewhat.
See the section: "Voltage multiplier starting circuit" for a description of
its design and operation.
Q1, ZD1, R8, and R11 form the low-side current regulator. The tube current
will be (15-.7)/R11 or just about 4 mA. So, for a different current, select
R11 to be 14.3/I.
Since the voltage compliance range of this power supply is only around 500 V,
the ballast resistor will still need to be selected carefully to achieve stable
regulation for your particular tube. See the section: "Identifying the range
of a power supply and selecting the ballast resistor".
ENHANCEMENTS TO AEROTECH MODEL PS1 HeNe POWER SUPPLY
Since the component values are all quite conservative, it should be possible
to safely boost the output of this supply by driving it with a Variac that
will go to 140 VAC. This will result in up to 2,400 VDC - enough to power
most laser tubes of up to 5 mW.
The modified circuit provides a current adjustment control, modulation input,
'Beam On' indicator, and tube current sense test points. I have implemented
these changes to the Aerotech PS1 and installed the current adjust pot, jacks
for Ground/Test+, Test-, Signal in, and Signal ground, and the Beam On LED on
the power supply case.
| (Remainder of circuit |Tube-
| identical to Aerotech PS1) +----+-----------+-------+---o +
| | _|_ | |
| | | | - ZD2 _|_ R13 / Test
| +----+ | 1N4742 /_\ 1K \ 1 V/mA
| | | | 12 V | /
| C6 +_|_ / R6 | | |
| 10 uF --- \ 510K | +-------+---o -
| 450 V - | / 1 W | __|__ IL2
| | | | _\_/_ Beam
| +----+ | | On
| | | | +---+
| C7 +_|_ / R7 | MJE2360T | C |
| 10 uF --- \ 510K | |/ |
| 450 V - | / 1 W | +---/\/\---| Q1 |
| | | | T2 | R15 |\ |
| +----+ Z +--+ + 15K | E |
| | | )||( | |
| | / R8 )||( | / R12
| | \ 470K )||( | \ 375 K
| C8 +_|_ / 1 W Signal in o----+ + / / 2 W
| 10 uF --- | 1:1 | R11 \ |
| 450 V - | +------------------------------+ 1.5K / |
| | | | |
| | ZD1 _|_, R14 / |
| | 1N4744 '/_\ 5K +->\ |
| | 15 V | Adjust | / |
| | | | | |
+---|<|---+-----------------------------------+---------+--+---+---o HV-
CR2
Each of the new and improved features is described below:
* To provide adjustable current, R11 is replaced with a fixed and variable
resistor in series. Using a 1.5K resistor and 5K potentiometer results in
a current range of approximately 2.2 to 9.5 mA. A heat sink should be used
on Q1 as it may be dissipating up to 5 W at maximum current and maximum
voltage (this power is higher for certain settings than in the original
Aerotech design).
* The Beam On indicator (IL2) is a high brightness LED in series with the tube
so it will glow whenever current is flowing. Its brightness provides a rough
indication of tube current as well.
* The test points permit the use of a multimeter to monitor beam current.
Using a voltage setting, the sensitivity is 1 mA/V. Current can also be
monitored directly between the test points (the current in R13 will be
negligible). ZD2 protects the multimeter should the sense resistor go
open-circuit for some reason :-(.
* The modulation input (Signal in) is coupled via T2, a small audio transformer
to provide HV isolation and permit the Tube- terminal to be earth ground.
The insulation rating on T2 should be at least 1,000 V. Assuming a 1:1
transformer, current sensitivity (percentage change of current with respect
to voltage) is about 7%/V relative to the set-point. The maximum input
should be limited to about 14 V p-p which will result in a current between
about 50% and 150% of the set-point. Thus, adjust R12 for 67% of the nominal
tube current with no signal. If the tube goes out on the negative peaks,
increase the set-point and decrease the amplitude of the input.
The phone line coupling transformer from a long forgotten 2400 baud modem
served nicely for this application resulting in a useful frequency response
from about 100 to 10,000 Hz.
* Another desirable enhancement (not shown) would be to provide a selection
of ballast resistor values in increments of 20K or 25K up to 150K. In
conjunction with the current control and optional Variac, this will provide
additional flexibility in matching the tube, supply voltage, and modulation
capabilities resulting in a 'universal' HeNe power supply.
With a small HeNe tube requiring about 1,200 V at 4 mA and additional 33K 5 W
ballast resistor, it was possible to adjust/modulate the current between about
2 and 6 mA. For testing, I used a Heathkit audio signal generator to drive
the modulation input and the simple circuit described in the section: "IR
detector circuit" with a scope across the C-E leads of the transistor as a
receiver. While this IR detector design is not really very good for linear
operation, with a little care in positioning the photodiode with respect to
the beam reflected off of a piece of paper, it was possible to display the
received signal on an oscilloscope. One could clearly observe the effects
of adjusting the current set-point and modulation signal amplitude and of
modulating beyond the rated tube current - the signal inverted (due to reduced
optical output power).
Stay tuned for exciting future developments.
A similar approach can be used with any of the other HeNe power supply designs
described in this document which use low-side regulation or which do not have
any regulation. Caution: don't try this with power supplies using high-side
regulation either by modifying the regulator (you would need a 15 KV coupling
capacitor or 15 KV opto-isolator to hold off the starting pulse) or adding an
additional low-side modulator (the two circuits will be fighting each other).
AEROTECH MODEL PS2B HeNe POWER SUPPLY
This one is definitely for higher power tubes. However, the basic design
is quite similar to those preceding. The estimated operating voltage is
3,600 V at 5 to 9 mA with a starting voltage of over 15,000 V. It includes
positive (anode) side regulation using an LM723 IC and a cascade of high
voltage transistors.
There may have been several versions of this model as I have two slightly
different samples using the same circuit board. The one described below which
designate model PS2B uses the higher voltage tap on the transformer. A nearly
identical design - model PS3A - runs with a transformer secondary of 1,150
VRMS yielding 3,000 VDC operating, 12,000 VDC starting, and uses only 8
electrolytic filter capacitors.
See the section: "Aerotech model PS2A-X HeNe power supply" for its circuit
diagram with my modifications.
Estimated specifications (Aerotech PS2B):
Operating voltage: 3,600 V.
Operating current: 5 to 9 mA.
Starting voltage: greater than 15,000 V.
Compliance range: 2,800 to 3,600 V at top of ballast resistor.
X R11 C11 C13 C15 C17
+---/\/\----||----+-------||------+-------||------+-------||------+
| 10M, 5 W CR3 | CR4 CR5 | CR6 CR7 | CR8 CR9 | HV+
| +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
| | | | | |
| | +----||----+-------||------+-------||------+ |
T1 | CR1 |Y | C12 C14 C16 |
+---+---|>|---+----+ +----+----+
||( | | | | |
||( | C1 +_|_ / R1 R12 / |
||( | 10 uF --- \ 510K T1: 1,380 VRMS, 20 mA 62K \ |
||( | 500 V - | / 1 W (primary not shown) 2 W / |
||( | | | | |/ C Q1
||( | +----+ CR1-CR9: (5 KV) +--| MJE2360T
||( | | | | |\ E
||( | C2 +_|_ / R2 C1-C10: 10 uF, 500 V R13 / |
||( | 10 uF --- \ 510K C11-C17: .005 uF, 5 KV 62K \ |
||( | 500 V - | / 1 W 2 W / |
||( | | | R1-R10: 510K | |/ C Q2
||( | +----+ R11-R14: 62K, 2 W +--| MJE2360T
||( | | | | |\ E
||( | C3 +_|_ / R3 Q1-Q3: MJE2360T R14 / |
||( | 10 uF --- \ 510K 62K \ |
||( | 500 V - | / 1 W U1: LM723 2 W / |
||( | | | R24 | |/ C Q3
||( | +----+ +---/\/\---+--| MJE2360T
||( | | | | 3.3K | |\ E
||( | C4 +_|_ / R4 | |/ E |
||( | 10 uF --- \ 510K +--------| Q4 | 2N4126
||( | 500 V - | / 1 W | |\ C | (PNP)
||( | | | | C18 | |
||( | +----+ +-------------------+----||----+----+
||( | | | | | .005 uF
||( | C5 +_|_ / R5 _|_, ZD1 |
||( | 10 uF --- \ 510K '/_\ 1N4744 +----------------------+
||( | 500 V - | / 1 W | 15 V, 1 W |
||( T | | | | |
+---|---------+----+ | R15 15K 1N4148 |\ | C
| | | | +--/\/\--+----------|+ \* |/*
| C6 +_|_ / R6 | +------+ |R14 15K | D1 |Err >--|
| 10 uF --- \ 510K | | Vref*|--+--/\/\------+---+--|- / |\ E
| 500 V - | / 1 W | +------+ 7.15 V | | | |/ |
| | | | | | | R21 /
| +----+ | +---+ | \ R16 10K \
| | | | | | _|_ / 82K /
| C7 +_|_ / R7 | C19 _|_ / /_\ \ ZD2 |
| 10 uF --- \ 510K | .1 uF --- \ | | 1M4733 _|_,
| 500 V - | / 1 W | | / | | 5.1 V '/_\
| | | | | | | | |
| +----+ +----+------------+---+---+----------------+
| | | | R17 15K |
| C8 +_|_ / R8 | R20 R19 |
| 10 uF --- \ 510K +-+-/\/\-----/\/\--------+-------+
| 500 V - | / 1 W | | 1.5K 1.8K |
| | | +---+ R25 /
| +----+ Current adjust: 6 to 11 mA 47K \
| | | 5 W /
| C9 +_|_ / R9 Note: Components marked |Tube+
| 10 uF --- \ 510K with '*' are part of .-|-.
| 500 V - | / 1 W U1, LM723. (Compensation | | |
| | | not shown.) | |
| +----+ Z | | LT1
| | | | |
| C10 +_|_ / R10 | |
| 10 uF --- \ 510K ||_||
| 500 V - | / 1 W '-|-'
| | | R23 |Tube-
+---|<|---+----+-------------+--/\/\--+------------------------+---o HV-
CR2 | 1K | _|_
- o Test o + -
1 V/mA
Note: the laser head itself may have an additional ballast resistor (not
shown).
The 1,380 V transformer output feeds a voltage doubler consisting of rectifiers
CR1 and CR2 and filter capacitors C1 through C10 resulting in about 3,600 V
across all the electrolytics. (Slightly less than 2 times the peak value of
1,380 VRMS.)
CR3 to CR9 and C11 through C17 form a classical voltage multiplier ladder
which generates a peak starting voltage of up to 2 * V(peak) + 8 * V(peak)
or 10 * 1,800 = 18,000 V but losses in the diode-capacitor network probably
reduce this somewhat. See the section: "Voltage multiplier starting circuit"
for a description of its design and operation.
Q1 through Q4, their associated resistors, and U1 (LM723) maintain a constant
voltage of 22 V across the combination of R19 + R20 so the tube current will
be 22/(R16 + R17). For example, with the R17 set for 750 ohms, the tube
current will be 6.3 mA. The adjustment range is approximately 5 to 9 mA.
The voltage compliance range of this power supply is about 800 V at 5 mA
(possibly a couple hundred volts greater at higher currents).
AEROTECH MODEL PS2A-X HeNe POWER SUPPLY
This is the other version of the Aerotech model PS2. I have modified it by
replacing the original (fried) high side regulator (identical to the one in
the model PS2B) with a wide compliance low side regulator using PNP
transistors instead of the more conventional NPN type. The advantage of using
PNPs is that the controls can be near ground potential (rather than floating
at the top of the transistor cascade) and mounted directly to the metal case.
As drawn, the compliance is about 800 V. The poor little panel mount pots
might not be very happy with that sort of voltage on them!
Estimated specifications (Aerotech PS2A-X):
Operating voltage: 3,000 V.
Operating current: 3 to 9 mA.
Starting voltage: greater than 12,000 V.
Compliance range: 2,200 to 3,000 V at top of ballast resistor.
X R11 C11 C13 C15 C17
+---/\/\----||----+-------||------+-------||------+-------||------+
| 10M, 5 W CR3 | CR4 CR5 | CR6 CR7 | CR8 CR9 | HV+
| +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
| | | | | |
| | +----||----+-------||------+-------||------+ /
T1 | CR1 |Y | C12 C14 C16 \ Rb
+---+---|>|---+----+ /
||( | | | T1: 1,150 VRMS, 20 mA |Tube+
||( | C1 +_|_ / R1 (primary not shown) .-|-.
||( | 10 uF --- \ 510K | |
||( | 500 V - | / 1 W CR1-CR9: (5 KV) | |
||( | | | | |
||( | +----+ C1-C4, C6-C9: 10 uF, 500 V LT1 | |
||( | | | C11-C17: .005 uF, 5 KV | |
||( | C2 +_|_ / R2 ||_||
||( | 10 uF --- \ 510K R1-R4, R6-R9: 510K '-|-'
||( | 500 V - | / 1 W RX1-RX3: 100K, 2 W |Tube-
||( | | | |
||( | +----+ QX1-QX3: MPSU60 +---------+-------+-+--o +
||( | | | _|_ | |
||( | C3 +_|_ / R3 - ZD2 _|_, R12 / Test
||( | 10 uF --- \ 510K 1N4742 '/_\ 1K \ 1 V/mA
||( | 500 V - | / 1 W 12V | /
||( | | | Beam On | |
||( | +----+ +-----------+---|<|--------+-------+----o -
||( | | | | | IL2 LED R13 R14
||( | C4 +_|_ / R4 | | +---/\/\---/\/\---+
||( | 10 uF --- \ 510K | | | | 5K 1.5K |
||( | 500 V - | / 1 W | +----+----+ Range |
||( T | | | | | | |
+---|---------+----+ | | | Q1 +----+
| | | | | | 2N3904 | |
| C6 +_|_ / R6 | ZD1 _|_, \ (NPN) |/ C |
| 10 uF --- \ 510K | 1N4744 '/_\ /<---------| |
| 500 V - | / 1 W | 15V | \ R15 |\ E |
| | | | | | 500K | |/ E QX1
| +----+ | | | Adjust +--| MPSU60
| | | | | | | |\ C (PNP)
| C7 +_|_ / R7 | +----+----/\/\----+ |
| 10 uF --- \ 510K | | R16 10K |
| 500 V - | / 1 W / R17 | |
| | | \ 100K | RX1 |/ E QX2
| +----+ / +----/\/\----+--| MPSU60
| | | | 100K | |\ C (PNP)
| C8 +_|_ / R8 | 2 W RX2 / |
| 10 uF --- \ 510K | 100K \ |
| 500 V - | / 1 W | 2 W / |
| | | | | |/ E QX3
| +----+ _|_ C18 +--| MPSU60
| | | --- 100 pF | |\ C
| C9 +_|_ / R9 | RX3 \ |
| 10 uF --- \ 510K | 100K / |
| 500 V - | / 1 W | 2 W \ |
| | | | | |
+---|<|---+----+-------------+-----------------------------+----+--o HV-
CR2
Note: the laser head itself may have an additional ballast resistor (not
shown).
The 1,150 V transformer output feeds a voltage doubler consisting of rectifiers
CR1 and CR2 and filter capacitors C1 to C4 and C6 to C9 resulting in about
3,000 V across all the electrolytics. (Slightly less than 2 times the peak
value of 1,150 VRMS.)
CR3 to CR9 and C11 through C17 form a classical voltage multiplier ladder
which generates a peak starting voltage of up to 2 * V(peak) + 8 * V(peak)
or 10 * 1,500 = 15,000 V but losses in the diode-capacitor network probably
reduce this somewhat. See the section: "Voltage multiplier starting circuit"
for a description of its design and operation.
Current adjust (R15) and current range (R13) pots have been added, the latter
being set by a screwdriver. This allows fairly linear control of tube current
up to the set limit from the front panel. The minimum current is determined
by what bypasses the transistors and passes through the base resistors. This
will be up to 3 mA depending on operating conditions.
As desribed in the secton: "Enhancements to Aerotech HeNe Power Supply PS1",
a current test point and 'Beam On' indicator have also been installed.
The NPN transistor (Q1) buffers the reference voltage so that the very low
current source from R15 can drive the base of the pass transistor cascade.
The base resistors, RX1 through RX3 equally distribute the voltage across the
3 PNP pass transistor, QX1 to QX3. The respective transistors act as emitter
followers and maintain approximately the same voltages across their C-E
terminals. Within the compliance range, the voltage across R13+R14 will be
nearly equal to the voltage on the wiper of R15.
R17 and C18 act as a snubber to protect the transistor cascade from the initial
over voltage when the tube fires but before the regulator can turn on. I do
not know whether this is needed or how much if any it would protect the pass
transistors when operating near their maximum ratings.
Three pass transistors are shown here only because that particular number fit
conveniently into the drawing :-). A greater or fewer number could be used
with their associated base resistors. I will probably use 4 to provide a
greater compliance and permit the same supply to drive a wider range of tubes.
If only one particular tube is to be driven, a single stage in conjunction
with a ballast resistor selected to set the operating current at the mid point
of the range may be adequate.
SAM'S LINE POWERED HeNe LASER POWER SUPPLY
This one is quite similar to the two Aerotech models PS1 and PS2 but is
constructed entirely with parts that are readily available and inexpensive.
Well, that is, except for the power transformer which you will still have
to scrounge from somewhere. See the section: "AC line operated power
supplies" for possible sources for these boat anchors. Also, due to low
demand, the prices of high voltage electrolytic capacitors seem to be quite
high (about $1.00 each for 10 uF at 450 V). I had a pair of surplus 1 uF,
1,500 V oil filled capacitors so I used them instead. The cost of the
remaining components (diodes, capacitors, and resistors) was less than $5.
* The high voltage rectifiers for the doubler are each constructed from five
1N4007s in series.
* The main filter on the doubler is a pair of 1 uF, 1,500 V oil filled
capacitors with 10M bleeder resistors on each.
* The high voltage rectifiers for the multiplier are each constructed from four
1N4007s in series.
The high voltage capacitors for the multiplier are each constructed from four
.001 uF, 1,000 V ceramic disk capacitors in series.
The series resistor for the parasitic multiplier is 10 M.
* There is currently no regulator - I may add that at a later time. For now,
a Variac is used to adjust beam current.
I have left room for equalizing components on the diode and capacitor stacks
but so far am running without them without any problems up to 2,500 VDC for
the operating voltage.
It took me roughly 3 hours to construct the doubler and starting multiplier on
an old blank digital (DIP) prototyping board.
I then tested it with a Variac and a current meter with several tubes from
1 mW to 5 mW:
* 1 mW Aerotech (4 mA): 1,900 V with Rb=100K, 1,700 with Rb=22K (additional
ballast resistor in laser head).
* 1 mW Spectra Physics (3.2 mA): 1,400 V with Rb=100K.
* 5 mW Aerotech (6 mA): 2,300 V with Rb=22K (additional ballast resistor in
laser head.
The Variac was quite effective at adjusting tube current.
At 115 VAC the output of the power supply is about 2,500 VDC. This design
appears to behave in all respects similarly to the commercial power supplies.
Estimated specifications (Sam-L1):
Operating voltage: 1,500 to 2,500 V.
Operating current: 0 to 10 mA.
Starting voltage: 7,500 to 12,500 V.
Compliance range: NA - no regulator as yet.
X R3 C3 C5 C7 C9
+----/\/\----||----+-------||------+-------||------+-------||------+
| 10M, 1 W CR3 | CR4 CR5 | CR6 CR7 | CR8 CR9 | HV+
| +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
T1 | CR1 |Y | | | |
+---+----|>|---+----+----||----+-------||------+-------||------+ |
||( | | | C4 C6 C8 |
||( | C1 _|_ / R1 |
||( | 1 uF --- \ 10M T1: 900 VRMS, 100 mA |
||( | 1,500 V | / (primary not shown) (1,9) R3 /
||( | | | 47K \
+---|----------+----+ CR1-CR2: (5 KV) (2) 5 W /
| | | CR3-CR9: (4 KV) (3) \
| C2 _|_ / R2 C1-C2: 1 uF, 1,500 V, oil filled |
| 1 uF --- \ 10M C3-C9: 250 pF, 4 KV (4) |
| 1,500 V | / LT1 |
| | | IL2 LED R4 Tube- +-------------+ Tube+ |
HV- o--+----------+----+----|<|---+---/\/\---+---+--|-| -|-------+
Beam On | 1K | _|_ +-------------+
o - Test + o -
Notes for Sam's line powered HeNe laser power supply:
1. T1 is from (approximately 40 year) old tube type TV. By using the lowest
line voltage tap and its 5 V and 6.3 V filament windings anti-phase in
series with the line input, its output has been increased from about 750
VRMS to 900 VRMS.
2. CR1 and CR2 each consist of five 1N4007s in series:
o--|>|--|>|--|>|--|>|--|>|--o
3. CR3 through CR9 each consist of four 1N4007s in series:
o--|>|--|>|--|>|--|>|--o
4. C3 through C9 each consist of four .001 uF, 1,000 V ceramic disc capacitors
in series:
o--||--||--||--||--o
5. Construction is on a blank digital prototyping board which just has pads
for 28 DIP locations (16 pins each). Perforated or other insulating board
could have been used as well.
6. Variac is used to adjust current - I will eventually add a low side
regulator similar to the one described in the section: "High compliance
cascade regulator".
7. Output is about 2,500 VDC at 115 VRMS input and 3,000 VDC at 140 VAC input.
8. There is audible evidence of HV breakdown near maximum output before the
tube starts. I suspect this is on the board itself since I have not coated
it as yet with HV sealer. This is not surprising since the output can
exceed 10,000 V.
9. Warning: the power transformer is capable of much more than the 20 mA
required for even higher power HeNe laser tubes making it particularly
dangerous - take extreme care not to touch (or even go near) the high
voltage terminals of this or any other high voltage power supply.
SIMPLE INVERTER TYPE POWER SUPPLY FOR HeNe LASER
This 2 transistor inverter is capable of driving a variety of medium to high
voltage applications from a 6 to 12 V, 2 to 3 A DC power supply, or auto or
marine battery. I have used variations of this basic circuit to generate
over 12,000 V for high voltage discharge experiments, test flyback (LOPT)
transformers, and power normal and cold cathode fluorescent tubes.
Here, the general design has been customized for use with small (.5 to 5 mW)
HeNe laser tubes requiring between about 1,100 and 2,000 VDC at 3 to 6 mA
(and possibly higher).
The inverter drive and multiplier starting circuits (if used) are similar to
plans for a small HeNe power supply found in the book: "Build your own working
Fiberoptic, Infrared, and Laser Space-Age Projects", Robert E. Iannini, TAB
books, 1987, ISBN 0-8306-2724-3.
However, with the designs below, all parts should be available without being
tied to the supplier listed in the book (Information Unlimited, assuming they
still even have these parts). However, there is something to be said for not
having to modify or wind your own transformer!
Also see the section: "Sam's inverter driven HeNe power supplies" for a way
to use this inverter design without a separate starting circuit.
SIMPLE INVERTER TYPE POWER SUPPLY SCHEMATIC
Two alternatives are described. These differ primarily in the details of the
high voltage secondary winding, rectifier/filter components, and whether a
separate starting circuit is required:
1. The transformer is totally custom but well specified using the core from
a small B/W TV or monochrome computer monitor flyback transformer. Three
sets of windings are added but this is not really difficult - just slightly
time consuming for the 1800 turn output winding if you don't have a coil
winding machine. Since the output is relatively high voltage, some care in
distributing and insulating the wire will be necessary.
Lower voltage rectifiers and filter capacitors can be used but a separate
starting circuit (e.g., voltage multiplier) will be needed for all tubes.
See the section: "Starting circuit for simple inverter type power supply
for HeNe laser" for a multiplier type starting circuit for this system.
2. The transformer is constructed using the core and high voltage secondary
(intact) from a small B/W TV or computer monitor flyback transformer. Two
sets of windings are added but these are only a few turns each. Note: the
flyback must *not* have an internal high voltage rectifier. If the primary
windings are not shorted, they can be ignored.
As an added bonus, with the flyback's HV secondary, there may be no need
for a separate starting circuit. Since it will have 3,000 or 4,000 turns
(compared to 1,800 turns for your homemade high votlage winding), the
no-load voltage will be much greater and should provide enough output
for tubes requiring less than about 8 KV starting voltage. Higher voltage
rectifiers and filter capacitors are required but construction is greatly
simplified by the elimination of the starting circuit. Where greater
starting voltage is required, a smaller multiplier (2 or 3 stages) will
likely be sufficient.
This is far and away the easiest approach since no tedious and time
consuming thousand+ turn coil winding is then required. I recommend you
try this first as it will save a great deal of time and effort.
See the section: "Sam's inverter driven HeNe power supplies" for details on
a high compliance design requiring no separate starting circuit.
The basic design including all primary side components is identical for both
approaches. The schematic shows D3, D4, C1, C2, specifically for the custom
wound HV winding (1) above and described in the text which follows.
+Vcc o T1 (1) X
o Q1 +----------------+ o
| | )|:| | D3
| B |/ C )|:| +----+----+----|>|----+-----o Y
| +---+----| 2SC1826 )|:|( | 3 KV (3) |
| | __|__ |\ E D 15T )|:|( | |
| | _/_\_ _|_ #26 )|:|( | |
| | _|_ - )|:|( HV 1800T | _|_ C1
| | - D1 1N4148 )|:|( #36 (1a) | --- .05 uF
+--|---------------------------+ |:|( | | 2 KV (4)
| | _-_ D2 1N4148 )|:|( | |
| | __|__ _-_ )|:|( T | |
| | _\_/_ | )|:| +---------------------+-----o Z
| | | B |/ E D 15T )|:| | |
/ | +----| 2SC1826 #26 )|:| | |
R1 \ | | |\ C )|:| | |
1K / | | | )|:| | _|_ C2
\ | | Q2 +----------------+ |:| | --- .05 uF
| | | |:| | | 2 KV (4)
| | | o |:| | |
| | +-----------------------+ |:| | D4 |
| | F 10T )|:| +----|<|----+-----o G
| | R2 100, 1 W #32 )|:| 3 KV (3)
+--+---------/\/\/\------------+
Windings: HV = High Voltage, D = Drive. F = Feedback.
(Values of C1, C2, D3, D4 shown design using custom wound HV winding.)
NOTES ON SIMPLE INVERTER TYPE POWER SUPPLY FOR HeNe LASER
1a. T1 is constructed on the ferrite core of a small B/W TV or monochrome
computer monitor flyback transformer or one that is similar. If using
a salvaged flyback, remove the core clamp or bolts and separate the core
pieces. Save the plastic core gap spacers for later use.
1b. It may be possible to use the high voltage secondary intact if it is in
good condition. However, the flyback must *not* have an internal high
voltage rectifier if a doubler (may be required to achieve sufficient
output for a high compliance design) is used for the operating voltage
or multiplier type starting circuit is used.
1c. The D (drive) and F (feedback) windings for T1 are wound using appropriate
size magnet wire (if available - hookup wire will work in a pinch) close
to the core. If possible, these should go on first *under* the high
voltage secondary. If not, wind them on the opposite leg of the core.
1d. Insulate the core and then wind the D and F windings adjacent to each
other. Bring the coil ends and centertap out one end and insulate them
from the windings they cross. Make sure all the turns of each winding
are wound in the same direction (both halves).
1e. If you are using the original HV winding, depending on its original
construction and whether you extracted the internal primary windings, it
may slip over the D and F windings.
1f. If you are adding your own HV (high voltage) winding, use a close fitting
plastic or cardboard tube or roll of paper on top of the primary windings
to provide a smooth uniform insulating form for the O winding.
Build up the 1,800 turn HV winding in multiple layers of about 200
turns where each is a single layer of wire. Use thin insulating (mylar)
tape between layers. Make sure the start and ends of this winding are
well insulated from all windings, the core, and everything else. Wrap the
outside with electrical tape to insulate it as well.
1g. Reassemble the core with its plastic spacers or add your own. With a core
air gap of .25 mm, the switching frequency is about 10 KHz. Selecting the
core gap size is one means of fine tuning operation.
2. The transistors I used were 2SC1826s but are not critical. Others such as
the common 2N3055 or MJE3055T types should also work. Any fast switching
NPN power transistor with Vceo > 100 V, Ic > 3 A, and Hfe > 15 should work.
For PNP types, reverse the polarity of the power supply and D1 and D2.
For continuous operation at higher power levels, a pair of good heat sinks
will be required.
3. Diodes D3 and D4 must be at least 3 KV PIV for an 1,800 turn HV winding
or 10 KV PIV using the original flyback's HV secondary. Fast recovery
types would be better but normal rectifiers seem to work. If diodes with
the required PIV rating are not available, build them up from 1 KV diodes
paralleled with 10 M resistors to balance the voltage drops. For testing,
I have been simply using strings of 1N4007s without apparent problems.
Your mileage may vary. Some commercial power supplies seem to omit the
equalizing components as well and get away with it. See the section:
"Edmund Scientific HeNe power supply".
4. Capacitors C1 and C2 must be at rated at least 1.5 KV for an 1,800 turn
HV winding or 5 KV using the original flyback's HV secondary. Where
capacitors with these ratings are not available, construct them from
several lower voltage capacitors in series with 2.2 M resistors to
balance the voltage drops due to unequal capacitor leakage.
5. Some experimentation with component values may improve performance for
your application.
6. When testing, use a variable power supply so you get a feel for how much
output voltage is produced for each input voltage. Component values are
not critical but behavior under varying input/output voltage and load
conditions will be affected by C1, the number of turns on each of the
windings of T1, the gap of the core of T1, and the gain of your particular
transistor. If the circuit does not start oscillating, interchange the
F winding connections to Q1 and Q2.
7. Add a post-regulator to this supply if desired to stabilize the current
since the inverter itself is not very well regulated.
8. WARNING: Output is high voltage and dangerous. Take appropriate
precautions.
9.
| | |
---+--- are connected; ---|--- and ------- are NOT connected.
| | |
STARTING CIRCUIT FOR SIMPLE INVERTER TYPE POWER SUPPLY FOR HeNe LASER
A voltage multiplier based design is shown. Other approaches can be used as
well - pulse trigger or wide compliance operation. See the chapter: "Helium
Neon Laser Power Supplies" and the section: "Sam's inverter driven HeNe power
supplies".
This is called a 'parasitic multiplier' since it feeds off of the main supply
and is only really active during starting when no current is flowing in the
HeNe tube. For details, see the section: "Voltage multiplier operation".
R1 C1 C3 C5 C7
X o---/\/\---||------+-------||------+-------||------+-------||------+
1M, 1 W D1 | D2 D3 | D4 D5 | D6 D7 |
+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+---o HV+
| | | |
Y o----------+-------||------+-------||------+-------||------+
C2 C4 C6
G o----------------------------------------------------------------------o HV-
X. Y, and G refer to the corresponding points on the schematic above or other
sample circuits in this document.
With 7 diodes, HV(peak) is approximately (X(peak) * 8) + Y and HV(average) is
(X(peak) * 7) + Y. For small tubes, fewer stages can be used. Increasing the
number of stages beyond what is shown may not boost output that much as the
losses due to diode and stray capacitance and leakage begin to dominate.
For the high frequency inverter, typical capacitor values are 100 pF.
The voltage ratings of the diodes and capacitors must be greater than the
p-p output of the inverter. The value of R1 can generally be increased to
10M without afffecting starting. A higher value is desirable to minimize
ripple in the operating current once the tube fires.
NOTES ON VOLTAGE MULTIPLIER STARTING CIRCUIT
1. Construction must take into consideration that almost 15 KV (in this case)
may be available at the output before the tube starts. Layout the circuitry
so that points with significant voltage differences are widely separated.
Perf. board or any other well insulated material can be used. A conformal
coating of clear high voltage sealer is recommended after the circuit has
been constructed and tested.
2. Diodes D1 to D7 must be rated at least 3 KV. Fast recovery types are
probably required. (The multiplier described in the section: "Sam's line
powered HeNe laser power supply" using normal 1N4007s does not appear to
generate adequate starting voltage when driven by this inverter). If 3 KV
diodes are not available, build them up from four 1 KV diodes (to add
margin if no equalizing resistors and capacitors are used).
3. Capacitors C1 to C7 are 100 pF 3 KV disk type.
SAM'S INVERTER DRIVEN HeNe POWER SUPPLIES
There are two variations on a similar approach which take advantage of the
high compliance/poor regulation of these inverters for starting. Thus, no
separate starting circuit is required.
These are both based on small flyback transformers and run on low voltage DC.
For this, I use a very basic transformer/rectifier/filter capacitor power
supply driven from a Variac.
No starting circuit is needed because of the wide compliance of thess circuits.
With no load (tube not lit), the voltage will climb to 5 to 8 KV or more. As
soon as the tube fires, the output drops to the sustaining ballast resistor
voltage for the operating current. In essence, the poor voltage regulation of
the inverter represents an advantage and allows this minimalist approach to be
effective.
This is one type of design where monitoring of the input voltage to the tube
is possible with a VOM or DMM requiring at most a simple high voltage probe.
Parasitic voltage multipliers may not have enough current capability and
trigger type starting circuits produce short high voltage pulses. It is
possible to clearly see the voltage to the ballast resistor/tube ramp up until
the tube starts and then settle back to its operating voltage. For small
tubes, I can safely use my Simpson 260 VOM on its 5 KV range without a high
voltage probe though it may go off scale momentarily.
The only additional components required for the HeNe laser power supplies may
be one or two high voltage rectifiers and a high voltage filter capacitor.
Since this is across the output at all times, it must be able to withstand the
starting voltage but be large enough to minimize ripple when the tube is
operating.
Caution: I would recommend using higher voltage capacitors than those shown
unless you know that your inverter is not capable of reaching the capacitor's
breakdown voltage. With some of these on a variable supply, 25 KV or more
open-circuit is quite possible due to wiring problems, no tube connected, a
bad or high starting voltage tube - or carelessness in turning the knob to far
clockwise!
I have also tried a 500 pF, 20 KV doorknob capacitor on design #2 (I didn't
have two such caps as required fro design #1). While this low value works,
it is a bit too small which results in about 20% ripple at an operating
voltage of 1,900 V and current of 4 mA with a 15 KHz switching frequency. The
minimal tube current setting for stable operation is slightly increased. At
lower switching frequencies it will be worse and may prevent the tube from
running stably at all A few of these caps in parallel would be better. Or,
use a stack of parallel plate capacitors made from aluminum foil and sheets
of 1/8" Plexiglas :-).
Warning: Since the voltage rating of these capacitors needs to be larger than
for power supply designs with separate starting circuits, it is possible for
a nasty charge to be retained especially if the tube should not start for some
reason. Stored energy goes up as V*V!
Note: The difference in energy stored in the filter capacitor between the
starting and operating voltages is dumped into the tube when it starts. For
long tube life this should be minimized. Therefore, a smaller uF value is
desirable for these high compliance designs. I do not know how much of an
issue this really represents. A post-regulator can be used to remove the
larger amount of ripple which results. However, such a regulator must have
overvoltage protection since at the instant the tube fires, it will see most
of the starting voltage.
SAM'S INVERTER DRIVEN HeNe POWER SUPPLY 1
This one is based on the inverter portion of the design described in the
section: "Simple inverter type power supply for HeNe laser" but using the small
B/W monitor flyback transformer option instead of a custom wound transformer.
(For the doubler, the flyback must *not* have an internal rectifier.) The only
differences are in the voltage ratings of the components required for the
doubler and filter capacitors (to the right of points X and T in that power
supply diagram).
Thus, it is an extremely simple circuit with no adjustments. Power output
is controlled strictly by varying input voltage. Only a pair of high voltage
rectifiers and a pair of high voltage filter capacitors for the doubler are
required to complete the power supply.
It requires between 6 and 12 VDC (depending on HeNe tube power and ballast
resistor) at less than 2 A and will power small HeNe tubes requiring up to
about 6 mA at 2,000 V, perhaps more.
Estimated specifications (Sam-I1):
Operating voltage: 1,200 to 2,000 V.
Operating current: 0 to 6 mA.
Starting voltage: 5,000 to 8,000 V.
Compliance range: NA - no regulator.
Here are sample operating points for two different 1 mW tubes:
Spectra Physics: (Rb=150K), Output: 1,450 V, 3 mA, Input: 6.25 VDC, 1.0 A.
Aerotech: (Rb=50K), Output: 1,500 V, 4 mA, Input: 7.0 VDC, 1.2 A.
Aerotech: (Rb=150K), Output: 1,900 V, 4 mA, Input: 7.5 VDC, 1.4 A.
The rectifiers (D3 and D4) should be rated at least 10 KV PIV (possibly higher
depending on the capabilities of your particular inverter). (However, don't
go excessively high as the voltage drop across the diodes could become rather
substantial.) In fact, when I replaced each of the high voltage rectifiers I
had been using with a string of 1N4007s, the tubes would run stably at slightly
lower output voltage (about 50 V less) and current.
The filter capacitor must be rated for the *maximum* no load voltage possible
with your inverter. For testing, I constructed it from two .25 uF, 4,000 V oil
filled capacitors in series with equalizing resistors providing about .12 uF
at 8 KV. With the components I used, the maximum no load output voltage was
slightly less than 8 KV with a 12 VDC input which is more than adequate to
start most smaller tubes. However, 10 KV capacitors should really be used.
+--------------+ X D3 Rb
Vin+ o-------| |---+-----|>|-----+-----+------/\/\-----+
| Simple | | | | 100K |
8 to 12 VDC, 2 A | Inverter | | C1 _|_ / R3 5 W |Tube+
| Power Supply | T | .25 uF --- \ 2.2 M .-|-.
Vin- o-------| |------+ 4,000 V | / | | |
+--------------+ | | | | | |
| +----------+-----+ | | LT1
| | | | |
| C2 _|_ / R4 | |
| .25 uF --- \ 2.2 M ||_||
| 4,000 V | / '-|-'
| | | R5 |Tube-
+-----|<|-----+-----+-----/\/\------+
D4 1K _|_
-
The tube current may be monitored as a voltage across R5 (1 V/mA) or directly.
It may be varied by adjusting the input voltage to the inverter. Using a
different ballast resistor value may also help to stabilize operation.
SAM'S INVERTER DRIVEN HeNe POWER SUPPLY 2
This inverter is the design from: "Adjustable high voltage power supply"
in the document: Various Schematics and Diagrams which additional info
about this circuit. Since I already had the inverter, it took a total of
about 10 minutes to convert it to a HeNe laser power supply!
It requires between 8 and 15 VDC (depending on HeNe tube power) at less than
2 A and will power small HeNe tubes requiring up to about 6 mA at 2,500 V,
perhaps more. With a 1 mW tube (1,900 V, 4 mA, 150K ballast resistor), the
input is about 8 VDC (probably about 1.5 A, not measured) and the switching
transistor heat sink doesn't even get warm :-).
Estimated specifications (Sam-I2):
Operating voltage: 1,200 to 2,500 V.
Operating current: 0 to 6 mA.
Starting voltage: 5,000 to 10,000 V.
Compliance range: NA - no regulator.
The schematics for the inverter are available in PostScript and GIF form. The
Postscript versions have been compressed PKZIP (DOS/Win3.1/Win95) and GZIP
(Unix).
hvgen32.ziphvgen32.ps.gzhvgen32.gif
The high voltage rectifier is built into the flyback transformer. If you have
to use an external rectifier, it should be rated at least 20 KV PIV (possibly
higher depending on the capabilities of your particular inverter). The filter
capacitors shown were just for testing. High voltage types are recommended,
again depending on the maximum output of your inverter with no load. For
testing, I constructed it from three .25 uF, 4,000 V oil filled capacitors in
series with equalizing resistors providing about .08 uF at 12 KV. However,
with the design as implemented, the maximum no load output voltage could
easily exceed 15 KV with a 15 VDC input.
+--------------+ HV+
Vin+ o-------| |---------------+-----+-------------+
| Adjustable | | | |
8 to 15 VDC, 2 A | High Voltage | C1 _|_ / R1 |
| Power Supply | HV- .25 uF --- \ 2.2 M / Rb
Vin- o-------| |---+ 4,000 V | / \ 150K
+--------------+ | | | / 5 W
| +-----+ |
| | | |Tube+
| C2 _|_ / R2 .-|-.
| .25 uF --- \ 2.2 M | | |
| 4,000 V | / | |
| | | | |
| +-----+ | | LT1
| | | | |
| C3 _|_ / R3 | |
| .25 uF --- \ 2.2 M ||_||
| 4,000 V | / '-|-'
| | | R4 |Tube-
+-----------+-----+-----/\/\----+
1K _|_
-
The tube current may be monitored as a voltage across R4 (1 V/mA) or directly.
It may be varied by adjusting the frequency and pulse width controls on the
inverter and its input voltage. Using a different ballast resistor value may
also help to stabilize operation.
It should be possible to add feedback from a current sense resistor to one of
the 555 timers to regulate output current by controlling switching frequency
or pulse width. This is left as an exercise for the student :-).
OTHER HeNe POWER SUPPLY CIRCUITS ON THE WEB
* The circuit at: http://www.execulink.com/~cake/LASERSUP.HTM is *supposed* to
run on 120 VAC but didn't have anything which looked like it would give the
starting voltage and besides, used a 9 V transformer *backwards* to provide
1,600 V. Yikes! and the words say a resistor passes 100 W!
My guess is that this never worked (at least not for more than a few
milliseconds) as it depends on putting 115 VAC into the 9 V winding of a
little transformer. These are not normally designed with any substantial
margins. It probably operates mostly in core saturation likely melting
down or blowing up or both in short order. Perhaps (generously) the author
was mistaken about the transformer or (more likely) never actually built
the thing at all.
I include a reference to it here only to warn that I do not recommend this
as a viable option.
* The circuit at:
http://www.ee.washington.edu/eeca/circuits/F_ASCII_Schem.html#ASCIISCHEM_007
is a 12 V switching supply, apparently identical to the design found in the
book by Robert E. Iannini (3).
As expected, there is no information on the special ferrite transformer
which they (the author of the book) expect you to obtain from Information
Unlimited. If it is still available, I expect the cost to be about $10.
See the section: "HeNe inverter type power supply" for details on a similar
design that you can build if you are willing to wind your own transformer.
ITEMS OF INTEREST - GENERAL LASER INFORMATION
WHAT MAKES A LASER POWER METER SO EXPENSIVE?
Commercial laser power meters cost $300 and up - $1,000 is a more typical
price for something that works over a wide range of power levels and
wavelengths. Where the precision and automatic wavelength calibration of
these instruments is not needed, a basic laser power meter can be built
inexpensively. See the section: "Sam's super cheap and dirty laser power
meter".
There are several ways to design a device that will determine the power in a
beam of light. Here are two:
* Photodiode - each photon within the wavelength range of the device creates
an electron-hole pair. When reverse biased, this results in a current flow
which is proportional to light flux.
* Thermo-electric - the beam hits a sensor that absorbs (nearly) 100 pecent
of the incident light energy at the range of wavelengths in question - a
black body. This raises its temperature with respect to a known reference
with a known thermal resistance between them.
Here are some comments on these approachs:
(From: Bill Sloman (sloman@sci.kun.nl)).
The important thing to note is that a photo-diode actually detects photons,
not power. Up to about 850nm, each photon actually reaching the diode junction
generates one pair of charge carriers. A 425nm photon, carrying twice the
energy of an 850nm photon generates the same pair of charge carriers, so the
same current represents the absorption of twice the power.
Since the 425 nm photon has rather less chance than the 850 nm photon of
actually surviving the trip down to the diode junction, so the actual ratio is
closer to 2.5:1.
Above 850 nm, the photons haven't got quite enough energy to separate a pair
of charge carriers, and can only separate those that are already somewhat
excited. The proportion that are sufficiently excited depends on temperature.
A electric field also helps, so biasing the diode increases it sensitivity to
long wavelength photons. As the wavelength rises above 850nm the extra energy
required to separate the charge carriers also rises, so the proportion of
'sufficiently excited' carriers declines quite rapidly.
In principle one could build a wavelength correction into the power meter,
but you would need to add a wavelength sensor to the power meter to make it a
useful feature.
The Centronics data book gives a typical spectral response for the 5T series
diodes, which effectively gives you the inverse of the wavelength correction
function, albeit with rather low precision.
The alternative approach is to use a sensor which responds to the heating
effect of the laser beam. These exist, but what you win on wavelength
independent calibration, you lose on sensitivity and zero stability - in
effect you have built a thermometer to measure the heating effect of your
laser beam on a more or less thermally insulated target. Unless someone has
done something very neat in this line, it doesn't strike me as a practical
proposition for your application, granting your limited budget.
SAM'S SUPER CHEAP AND DIRTY LASER POWER METER
Hobbyists and experimenters may not need the super precision or automatic
features of a commercial (and costly) laser power meter. For example, the
wavelength or wavelength distribution of the laser source is almost always
known. Therefore, if a correction needs to be computed using mushware (i.e.,
the stuff between your ears), so be it. There will be no absolute reference
either but calibration using a source with known output power and wavelength
like a 1 mW HeNe 632.8 nm laser will work just fine. And, if you really want
a 16 digit LCD display, one can always be added :-).
I tossed this together using a 4 segment photodiode chip from a dead and
abandoned Mouse Systems optical mouse (the old type which uses a pair of
these chips - one for each axis). The active area of each segment is about
1 mm x 1.4 mm (total about 1 mm x 5.6 mm) which isn't great but is adequate
to capture the entire beam of a typical collimated laser diode or HeNe laser.
A larger area photodiode would be better. To ease this a bit, I tied all 4
segments in parallel so one dimension is no problem at all. There are
microscopic gaps between the segments but I estimate it to be less than 5
percent of the area so the loss should not be a big problem.
An 'instrument' (this term is being used very generously!) of this type will
not replace a $1,000 commercial laser power meter but may be sufficient for
many applications where relative power measurements are acceptable and/or
where the user is willing to do a little more of the computation :-). One
cannot complain about the cost: $0.00.
The basic circuit is as follows:
R1 1K 1 2
Vcc o---------/\/\---------+----|<|----+
| 4 3 |
+----|<|----+ U1
| 5 6 | AE1004
+----|<|----+
| 8 7 |
+----|<|----+
M1 |
+---------+ |
- | 0-10 mA | + | PD
Gnd o------| \ |-----------------+
| o | <- I
+---------+
* The meter (M1) I used was a D'Arsenval moving coil type that had a full scale
sensitivity of 10 mA. A suitable shunt can be used with a more sensitive
meter or just use one of the current ranges of your VOM or DMM.
R1 provides current limiting to protect the meter movement from vaporization
should the photodiode array short out. The combination of Vcc and R1 just
needs to meet the requirement that the photodiode array remains reverse
biased at the maximum expected current (optical power).
For the value of R1 shown above, Vcc should be at least 4 VDC for a
photodiode current up to about 3 mA.
* I do not know the maximum ratings of this photodiode array but it seems to be
fine with Vcc up to at least 12 VDC. Since current is nearly independent of
the bias voltage, Vcc is not at all critical.
* Sensitivity is about .45 mA/mW at 632.8 nm from a HeNe laser. Though I have
nothing precise to calibrate it against, the readings were consistent and
linear with the tubes I tried which had their output power labeled.
* Mount the photodetector on a 'third hand' type of mount so it can be easily
positioned in the beam path.
* A lens can be used to reduce the beam diameter of your laser so that the
entire beam fits within the area of the photodetector. Where the beam
profile exceeds the dimensions of the photodetector, an estimate of beam
power can still be made knowing the ratio of sensor area to total beam area.
Unfortunately, with the small area of the photodetector, using this with
intact CD laser optics may not be that easy.
* The range of wavelengths over which this is useful should extend throughout
the visible spectrum into the near IR - at least until 850 nm or so.
I do not know what precise effect different wavelength lasers will have on
the sensitivity of this circuit. Shorter wavelengths are more energetic but
generate the same number of charge carriers (i.e., same current) and have
less chance of surviving the trip through the diode junction. Thus, for a
given photon flux the power reading will be low at shorter wavelengths. A
correction factor can probably be computed.
* I also do not have any idea at what point the photodiode array will be
damaged due to thermal effects. This is certainly not a problem for up to
10 mW as long as it is not focused to a sharp point.
A pair of op-amps can be added to provide more flexibility. The following
circuit is substituted for the meter (M1), above. Any general purpose op-amps
(e.g., 741) powered from +/- 12 VDC (for 10 V full scale) can be used.
R2 1.11K
+------/\/\------o X1
| R3 11.1K X10 S1 Range Select
+------/\/\----o <---o--+
| R4 100K |
+------/\/\---+--o X100 | R6 1K R7 5K Calibrate
| | | +---/\/\---/\/\---+
I-> | |\ | | | | |
PD o-----+---|- \ | | R5 1K | |\ +----+
| >----+---------+---/\/\---+---|- \ |
+---|+ / | >--------+----o +
_|_ |/ U2 +---|+ / Vout
- _|_ |/ U3 +--o -
- _|_
-
This circuit provides 3 ranges. R7 (calibrate) allows the sensitivity to be
adjusted for your particular photodiode and laser wavelength. With R7 set to
1.22 K, the ranges will be .01 mW, .1 mW, and 1 mW per V of Vout at 632.8 nm.
Vout can also be monitored with a scope or connected to an audio amplifier
to detect an amplitude modulated laser beam.
For the Range Select switch (S1), make-before-break contacts are recommended
to prevent high amplitude glitches when changing ranges.
For my photodiode array, the dark current was insignificant. Should this not
be the case with your device a potentiometer tied to a negative reference can
be used to null it out by injecting an equal and opposite current at the (-)
input to U2.
Many variations and enhancements to this circuit are possible.
ABOUT LASER SPECKLE AND OTHER PHENOMENA
Speckle is a mottled pattern that arises when laser light falls on a
non-specular reflecting surface. Lasers with high spatial and temporal
coherence properties are likely to produce dramatic speckle effects. Thus,
gas lasers like HeNe types are more likely to exhibit this effect than laser
diodes.
For those applications where the laser's bright light and its ability to be
sharply focused or easily collimated are important but coherence is irrelevant,
speckle is an undesirable side effect to be avoided.
(From: Mike Poulton (tjpoulton@aol.com)).
If you want any more information on any kind of laser, or sources for parts to
build them, post your question on alt.lasers. There, a group of about ten
laser enthusiasts (including myself) will jump on your question and answer it
in every possible way and in great detail.
As for the speckle pattern, that is usually called the interference pattern.
It has nothing to do with your eyes and has no bearing on how well you can see
as it is a real phenomenon. Laser light is completely monochromatic and is
also in phase. When this light is scattered, it gets out of phase and the
waves collide. When a wave at a low point and a wave at a high point collide,
they cancel each other out (just like those noise-reduction machines that send
out ambient sound 180 degrees out of phase, except this is with light). Where
the light cancels itself out, there is a dark space, where it does not, there
is a light space. This creates a three-dimensional lattice-work of light and
dark spaces.
As you move around it, you see different parts of the lattice and your view
appears to move. The more "saturated" the area is with light, the more
impressive this effect is. I have a 15mW Helium-Neon laser, and its effect is
incredible. To say that this is in your head is like saying that it is an
optical illusion when you look at different sides of a house. One cool thing
to try is shining the laser into flood light (while it is turned off). The
reflective coating on the inside of the bulb makes this effect very intense.
(From: J. B. Mitchell (ugez574@alpha.qmw.ac.uk)).
Speckle noise arises because of the highly coherent nature of the laser light
and can thus be reduced or eliminated by reducing the coherence of the source.
One easy way of achieving this is by introducing a rotating ground-glass screen
into the beam. Placing the ground glass at the focus of the beam reduces the
temporal coherence by introducing random phase variations while maintaining the
spatial coherence (ability for the beam to be focused to a point). Putting
the ground glass in an unfocussed beam reduces both the temporal and spatial
coherence.
Alternatively, if you need to maintain the coherence for your application
(interferometry, for example) the you can reduce the size of the speckles
by increasing the aperture of the imaging system.
(From: Steve McGrew (stevem@comtch.iea.com)).
I know of three ways:
1. Increase the spatial frequency of the speckle so that it is so high it
ceases to be a problem.
2. Use only specular objects and sources.
3. Decrease the temporal and/or spatial coherence of your laser beam by
running it through something like a rotating diffuser.
DIFFERENCE BETWEEN FABRY-PEROT AND DFB LASERS
The Fabry Perot laser design is what most people think of: lasing medium
with mirrors at each end.
(From: Dr. Mark W. Lund (lundm@acousb)).
A Fabry-Perot cavity is the standard run of the mill cavity with two highly
reflecting mirrors bouncing the light back and forth, forming a standing wave.
This cavity is not very frequency selective, theoretically you could have 1 mm
wavelength light and .001 micron wavelength light in the same cavity, as long
as the mirrors are the right distance apart to form a standing wave (and higher
order modes make this easier than you might think).
A distributed feedback laser replaces the back mirror with a grating along the
cavity axis. Instead of being reflected abruptly like a metal mirror would,
the grating reflects a little over each part of the grating until at the back
of the grating the light has petered out. Of course, since the light is being
reflected by the grating the reflected light is always in the correct phase
no matter if it was reflected from the front or back of the grating. The
distributed nature of the reflection sharpens the cavity resonance and
distributed feedback lasers are typically of much narrower bandwidth than the
same laser with mirrors. Mostly seen in laser diodes, distributed feedback
can also be done with non-linear optics, volume gratings, and other more
esoteric optical elements.
(From: Bret Cannon (bd_cannon@pnl.gov)).
Fabry-Perot lasers are made with a gain region and a pair of mirrors on the
facets, but the only wavelength selectivity is from the wavelength dependence
of the gain and the requirement for an integral number of wavelengths in a
cavity round trip.
DFB (Distributed Feed Back) lasers have the a periodic, spatially-modulated
gain, which gives a strong selectivity for the wavelength that matches the
period of the gain modulation. DFB lasers lase in the same single longitudinal
mode from threshold up to the maximum operating power while Fabry-Perot lasers
hop from one longitudinal mode to another as the current and/or temperature
change. Most Fabry-Perot lasers lase on several longitudinal modes
simultaneously though with some of these lasers you can find currents and
temperatures where they lase on only a single mode.
The are also DBR (Distributed Bragg Reflector) lasers that have a Bragg
reflector, that is a volume grating, as the reflector at one end of the
cavity, which provides wavelength selective feedback. These lasers lase
on a single longitudinal mode but the lasing hops from longitudinal mode
to longitudinal mode to stay near the peak of the reflectivity of the
Bragg reflector as temperature and current are changed.
COMMENTS ON VARIOUS COLOR LASERS
(From: Mike Poulton (tjpoulton@aol.com)).
Laser diodes have only been able to produce red and infrared beams so far
(at least commercially). There have been some research reports of green
and possibly blue laser diodes but only operating in pulse mode, at reduced
temperature, and/or with very limited lifetime. This will no doubt change
as enormous incentives exist to develop shorter wavelength laser diodes for
numerous applications.
The green lasers you see are either argon or frequency-doubled Nd:YAG
(neodymium doped yittrium-aluminum-garnet). The argon laser is a very
large and complex device, almost always putting out hundreds of times the
power of your pointer. A Nd:YAG laser is usually even more powerful, but
is often pulsed. Diode lasers are not used in laser light shows because
they are never powerful enough. I am sitting here typing this while
allowing my 15mW Helium-Neon laser to stabilize and warm up. Its
wavelength is shorter, and it is 3 times more powerful than the pointer.
When a red beam is needed in a laser light show, these are usually used
because they are usually more powerful than diodes, and the beam is more
visible per milliwatt because of it's shorter wavelength. Happy Lasing,
and be sure to visit alt.lasers for any laser info you need!
COMMENTS ON SPACIAL AND TEMPORAL COHERENCE
(From: Daniel Marks (dmarks@uiuc.edu)).
There are really two coherences associated with any source; spatial and
temporal coherence. Probably the coherence you are referring to is temporal
coherence, but both are important for holography.
The temporal coherence is related to the bandwidth of the source. The more
narrow the bandwidth of the source, the longer the coherence length. HeNe
lasers have a very narrow bandwidth, as a result they have a coherence length
on the order of 10-30 cm. LED's are incoherent sources, they only have a
coherence length of 10-40 microns, and a large bandwidth of several kT (25.9
meV at 298K) or I'm guessing 10 nm of bandwidth (around about 650 nm). HeNe
lasers are also much more spatially coherent than LEDs. The spatial coherence
length is determined by the cavity and cavity reflectivity in a laser. LEDs
also have a very short spatial coherence length, or only a couple of
wavelengths.
The coherence length is the maximum distance at which two points in the field
can be interfered with contrast. The temporal coherence length determines the
maximum depth of the object in a reflection hologram, and the spatial coherence
length determines the lateral size. Using techniques of "white light"
interferometry, incoherence sources can be used, but they are tricky and have
many restrictions on the kinds of holograms one can create.
LASER BEAM COLLIMATION
(From: Kai-Martin Knaak (kmk@physik.uni-mainz.de)).
There is a maximum distance that a beam of light can be kept collimated.
Usually it is called 'Rayleigh length' and it depends on the wavelength and
the minimum diameter of the beam. If the beam diameter is w0 at point z, then
the beam will have expanded to at least 1.4 times w0 at Rayleigh-length
distance from z.
The Rayleigh length, z_rayleigh, can be calculated like this:
z_rayleigh = pi * (minimum diameter)/(wavelength)
For example, assuming a HeNe laser (632.8 nm) and a minimum diameter of 6 mm
this makes about 180 meters. In practice, you might not get that far but 50
meters may be feasible. (Reality enters due to the fact, that the axial
intensity distribution is assumed to be perfectly gaussian.)
One way of doing the collimation, is with a telescope consisting of two
identical plano-convex lenses. If the lenses are spaced at the double focal
length, their effect onto beam divergence will vanish. Putting them nearer
increases divergence and moving them farther apart focuses the beam. So you
can collimate the beam by fine tuning the lens distance.
Of course lens aberrations limit the performance, so weak lenses or aspheric
lenses might be desirable. Spherical aberration will be reduced by turning
the curved sides of the lenses face to face.
See the book "Lasers" by A. E. Siegmann for the details of the propagation of
laser light. (page 664 ff.)
GENERAL COMMENTS ON LASERS AS A HOBBY
We have addressed the issues involved in using common laser diodes and HeNe
laser tubes. If you are really serious and want to go further, here are some
comments on a variety of lasers.
(From: Richard Alexander (RAlexan290@gnn.com)).
How much do you like to build things? Would you prefer to assemble a bunch of
parts, or do you want to blow your own glass tubes, too? Do you have any
mechanical experience? Do you build electronic kits? Keep in mind that you
will often be working with intense light (enough to instantly damage your
unprotected eyes, and maybe your unprotected skin) and high voltages.
All laser experimenters (and optics types, too) should have a copy of
"Scientific American"'s "Light and Its Uses." It gives construction plans for
a Helium Neon (you blow the glass tube yourself), an Argon Ion (even more
complicated), a CO2 (designed and built by a high school student, and able to
cut through metal), a dye, a nitrogen (a great first laser, but watch out for
UV light) and a diode laser (obviously, you buy the diode laser and assemble
the driver circuit from the plans they supply). They also explain how to make
holograms using visible and infrared light, microwaves and sound. There are
other projects, too. The book is getting fairly old (the HeNe dates to the
'60s or '70s), but it's still a great reference.
A nitrogen laser may be built for under $200 (maybe less than half that amount
if you are lucky). It requires no mirror alignment (since it has no mirrors).
The technology for building this laser was available to Ben Franklin, so there
is nothing too critical in it. The hazards it presents are lots of ultraviolet
light (spark discharges and laser beam), high voltage (necessary to arc across
a 1/4 inch spark gap in a nitrogen environment) and circuit etcher (the main
capacitor is made from an etch circuit board).
Once built, the nitrogen laser can drive many other projects. It can be used
as a pump for the dye laser, for example. It will light up anything
fluorescent. It is a pulse laser (10 ns) that can be repetitively pulsed (120
Hz is a likely frequency). Megawatt power is possible, but the total energy is
low (due to the short pulses).
"Electronics Now" (formerly, Radio Electronics) has a laser projects column
that started several months ago. I'm trying to think up a project I can submit
to them. They said they would welcome projects for the laser column.
Helium Neon laser tubes may be bought from many mail-order companies. I bought
one from Meredith Instruments in Arizona. They cost about $15, and the power
supply can be built or bought for about another $20. You have the option of
buying tubes with mirrors attached or not. You might want to buy the mirrors
attached, because aligning those mirrors is extremely tedious. I was given an
"A" for constructing a working Helium Neon laser from the parts in the Laser
Lab in less than an hour. The class was given two semesters to gain the
experience they needed to do that.
If you want more than one color from lasers, there are various ways to do it,
but none of them are as nice as one might like. For $3000 or so, you can buy a
Helium Neon laser that will produce laser light ranging from infrared to
blue. All you have to do is turn a dial on the back.
Laser light shows usually use Argon Ion or Krypton lasers. These are able to
produce most of the colors of visible light, and they can also be dialed to
the desired color. However, they usually cost several thousand dollars
($40,000 is not too unusual) and require either forced air or water cooling
or a combination.
LASER INFORMATION RESOURCES
REFERENCES ON LASER PRINCIPLES, TECHNOLOGY, CONSTRUCTION, AND APPLICATIONS
1. The Laser Cookbook: 88 Practical Projects
Gordon McComb
TAB Books Inc, 1988.
Blue Ridge Summit, PA 17214
2. Build your own Laser, Phaser, Ion Ray Gun & Other Working Space Age
Projects
Robert E. Iannini
TAB Books, a division of McGraw Hill, 1983
Blue Ridge Summit, PA 17214
ISBN 0-8306-0604-1 paperback
3. Build your own working Fiberoptic, Infrared, and Laser Space-Age Projects
Robert E. Iannini
TAB books, a division of McGraw-Hill, 1987
Blue Ridge Summit, PA 17214
ISBN 0-8306-2724-3
This includes plans for a HeNe power supply as well as complete ruby/Nd-YAG
and CO2 lasers and other interesting stuff.
4. Try the magazine Scientific American, the column Amateur Scientist, the
time period 1960-1980.
5. Light and its Uses
(readings from Scientific American) C. L. Strong's 'The Amateur Scientist'
with introductions by Jeral Walker.
W. H. Freeman And Co.
ISBN 0-7167-1184-2, ISBN 0-7167-1185-0 (pbk).
Info on how to build lasers and how to use them, as well as info on
building laser instruments.
All of John Strong's (genius experimentalist) and Jeral Walkers columns
on photonic devices are in this absolutely fabulous book.
The book describes the manufacture of several lasers by amateurs, form HeNe
through CO2 and Dye types. Also, some Hologram and interferometer stuff.
It is not for the absolute beginner but suitable for anyone who has some
considerable hobbyist type experience with electronics and/or lasers.
6. Some older issues of Popular Electronics and Radio Electronics have
articles on how to use HeNe lasers and power supplies for them (maybe
1980-1989).
7. Forrest Mims' Circuit Scrapbook II
Forrest Mims
Howard Sams & Co., 1987
This book is out of print but available at some libraries. It provides
various driver circuits and a miniature laser + driver + battery built
into a very small package.
Forrest Mims has also written a number of articles on how to use and build
lasers.
8. The Laser Book - A New Technology of Light
Clifford L. Lawrence
Prentice Hall Press, 1986
A division of Simon and Schuster
New York, NY 10023
ISBN 0-13-523622-3
This book includes descriptions of many common lasers, construction, and
applications.
9. Lasers and their Applications.
Kurt R. Stehling
The World Publishing Company, 1966
Cleveland and New York
Library of Congress Catalog Number: 66-18464
10.Introduction to Laser Physics
Bela A. Lengyel
John Wyley and Sons, Inc., 1966
New York, London, Sydney
Library of Congress Catalog Number: 65-27659
If you always wanted to really understand terms like population inversion,
hyperfine transitions, and quantum efficiency, this old but solid book is
for you. Be prepared for some heavy math. However, it does include some
practical aspects of laser construction as well.
11.Wedding Lasers to Power Supplies
Photonics Spectra, June 1982
This is a nice article on general power supply considerations for HeNe
and (small) CO2 lasers.
12.Another place you may try is "The Bell Jar" a newsletter on high vacuum
amateur work it sometimes includes laser information. They have a WEB
site: http://www.tiac.net/users/shansen/belljar/articles.htm.
13.Some of the earlier columns of "The laser Experimenter" (1995) went into
detail on how to make light shows, and how to construct the power supplies
for the HeNe type of lasers.
14.The March 1989 issue of Radio-Electronics magazine has plans for a HeNe
power supply running on 12 VDC using a 555 timer chip and two transistors,
a relay, and a 12 V to 280 V step-up transformer.
You may be able to find many of these items in a large public library. I
think that the old issues of magazines are often on microfilm or microfiche.
WEB RESOURCES
The following site will be jointly hosted by Ed Edmondson Jr. and myself:
http://www.geocities.com/CapeCanaveral/Lab/3931/
"Our mission here is to provide informtion about Lasers and Laser related
equipment to the Amateur, Hobbyist, and Experimenter. Our site is the focal
and collecting point of FREE information about these wonderful light emitting
devices and their technology."
A current version of this document will always be present in some form at
this site.
The following have various information and links to other laser related sites:
* http://www.misty.com/~don/laserdon.html (Don Klipstein's lighting center)* http://www.rli.com/ (Lasernet homepage)* http://www.rli.com/tutor1.html (Lasernet Laser tutorial)* http://www.achilles.net/~jtalbot/history/ (History of lasers)* http://www.rli.com/lazindex.html (Links to laser related sites)LASER PARTS SOURCES
WALK-IN:
* Radio Shack offers a variety of laser pointers which may be suitable for
various types of simple laser experiments. While they have a variety of
electronic components as well, don't expect to find those that you would
need for serious laser power supply construction.
* Edmund Scientific has a retail store located in Barrington, New Jersay.
Complete directions are at their web site (http://www.edsci.com/). This
is a must-see if you are in the area (and worth a detour if you are not).
* Some of the places listed in the section: "Mail order" also have retail
outlets.
MAIL ORDER:
It is well worth asking for catalogs and getting on the mailing lists of all
of these companies as they offer a wide variety of neat, nifty, and often
useful electronic, mechanical, and optical items often at excellent prices.
Offerings include new, used, or surplus laser components:
* Meredith Instruments 1-602-934-9387, http://www.mi-lasers.com/. Extensive
on-line catalog with prices for all items.
* MWK Industries 1-714-278-0563, http://www.mwkindustries.com/. Complete
on-line catalog with photos and prices of all items.
* Herbach & Rademan, 1-800-848-8001, http://www.herbach.com/. Includes some
HeNe tubes and power supplies, a few optical components, motors, and a
variety of other interesting and useful electronic parts. New arrivals and
closeouts are listed at web site, a quarterly catalog is available by
telephone, fax, or email request.
* Midwest Laser Products, 1-708-460-9595.
* World Star Technologies, 1-416-204-6298, http://arcos.org/laser/INDEX.HTM
Laser modules and laser pointers.
Electronics bargains, occasional lasers - varies from month to month:
* All Electronics, 1-800-826-5432, http://www.allcorp.com/
* Halted Specialties, http://www.halted.com/
* Mega Surplus, 1-314-291-7618, http://www.i1.net/~mega/
* Hosfelt Electronics, 1-800-524-6464
Other sources:
* Electronic Rainbow, 1-317-291-7262.
* Timeline Inc. 1-310-784-5488.
* DigiKey, 1-800-DIGIKEY, http://www.digikey.com/. A couple laser diode
modules and bare laser diodes. Electronics parts, of course.
Electronic and laser project parts, plans, specialized components:
* Information Unlimited/Amazing Concepts
Inquiries: 1-603-673-4730, Orders: 1-800-221-1705, Fax: 1-603-672-5406
E-mail: wako2@xtdl.com
Web: http://www.amazing1.com/
This place is definitely worth an 'at least check out their Web site'. Much
weird stuff including specialized parts (as well as plans and complete kits)
needed for the projects in the two Iannini books (2) and (3) (though cheaper
alternatives using readily available components may be available).
High quality (but expensive) lasers and optics as well as many bargain priced
new and surplus scientific items:
* Edmund Scientific
101 East Gloucester Pike
Barrington, NJ 08007-1380
Phone: 1-609-573-6250
Web: http://www.edsci.com/
Their catalog is a must even if you never intend to purchase anything. I
remember fascinating trips to their retail store stocked with bin-upon-bin
of interesting and unusual optical and electornic items. I do not know
what it is like these days.
COMPREHENSIVE LIST OF LASER DIODES AND SUPPLIERS
If you are really serious about diode lasers - not just the common $15 laser
pointer variety, contact ThorLabs and ask for "Thor's Guide to Laser Diodes",
published by ThorLabs, 75 Mill Street, P. O. Box 366, Newton, NJ 07860-1453.
Phone: 201-579-7227, Fax: 201-383-8406.
This is a free comprehensive list of commercially available diode lasers
arranged by wavelength.
SOME LASER MANUFACTURERS
* PMS Electro-Optics
1855 South 57th Court
Boulder, CO 80301
303-443-7100
FAX 303-449-6870
TWX 469-143 or 910-940-5891
* Melles Griot
2251 Rutherford Rd.
Carlsbad, CA. 92008
619-438-2131
FAX 619-438-5208
TWX 695631
WEB: http://www.mellesgriot.com/MG-HOME.htm